CHAPTER 3: What is the difference between a formula mass and a molecular mass? Could a given substance have both a formula mass and a molecular mass?
The molecular mass is the sum of the atomic masses of all the atoms in a molecule of the substance whereas the formula mass is the sum of the atomic masses of all the atoms in one formula unit of the compound, whether the compound is molecular or not. A given substance could have both a molecular mass and a formula mass if it existed as discrete molecules.
Describe in words how to obtain the formula mass of a compound from the formula.
To obtain the formula mass of a substance, sum up the atomic masses of all atoms in the formula of the compound.
One mole of N?molecules? How many N atoms are there in one mole of N2? One mole of iron (III) sulfate, Fe2(SO4)3, contains how many moles of SO4²? ions? How many moles of O atoms?
A mole of N2 contains Avogadro’s number (6.02 x10²³) of N2 molecules and 2 x 6.02×10²³ atoms. One mole of Fe2(SO4)3 contains three moles of SO4²? ions, and it contains twelve moles of O atoms.
Explain what is involved in determining the composition of a compound of C, H, and O by combustion
A sample of the compound of known mass is burned, and CO2 and H2O are obtained as products. Next, you relate the masses of CO2 and H2O to the masses of carbon and hydrogen. Then you calculate the mass percentages of C and H. You find the mass percentage of O by subtracting the mass percentage of C and H from 100.
Explain what is involved in obtaining the empirical formula from the percentage composition
The empirical formula is obtained from the percentage composition by assuming for the purposes of the calculation a sample of 100 g of the substance. Then the mass of each element in the sample equals the numerical value of the percentage. Convert the masses of the elements to moles of the elements using the atomic mass of each element. Divide the moles of each by the smallest number to obtain the smallest ratio of each atom. If necessary, find a whole-number factor to multiply these results by to obtain integers for the subscripts in the empirical formula.
A substance has the molecular formula c6h12o2, what is its empirical formula?
The empirical formula is the formula of a substance written with the smallest integer (whole number) subscripts. Each of these subscripts in the formula C6H12O2 can be divided by 2, so the empirical formula of the compound is C3H6O.
Hydrogen peroxide has the empirical formula HO and an empirical formula weight of 17.0 amu. If the molecular mass is 34.0 amu, what is the molecular formula?
The number of empirical formula units in a compound, n, equals the molecular mass divided by the empirical formula mass.
n = 34.0 amu / 17.0 amu = 2.00
The molecular formula of hydrogen peroxide is therefore (HO)2 or H2O2
Describe in words the meaning of the equation
CH4 + 2O2 ? CO2 + 2H2O
using a molecular, a molar and then a mass interpretation.
The coeffiecients in a chemical equation can be interpreted directly in terms of molecules or moles. For the mass interpretation, you will need the molar masses of CH4, O2, CO2, and H2O which are 16.0, 32.0, 44.0 and 18.0 g/mol.
2 x 32.0 g
Explain how a chemical equation can be used to relate the masses of different substances involved in a reaction.
A chemical equation yields the mole ratio of a reactant to a second reactant or product. Once the mass of a reactant is converted to moles, this can be multiplied by the appropriate mole ratio to give the moles of a second reactant or product. Multiplying this number of moles by the appropriate molar mass gives mass. Thus, the masses of two different substances are related by a chemical equation.
What is a limiting reactant in a reaction mixture? Explain how it determines the amount of product.
The limiting reactant is the reactant that is entirely consumed when the reaction is complete. Because the reaction stops when the limiting reactant is used up, the moles of product are always determined by the starting number of moles of the limiting reactant.
Come up with some examples of limiting reactants that use the concept but don’t involve chemical reactions.
Two examples are given in the book. The first involves making cheese sandwhiches. Each sandwhich requires two slices of bread and one slice of cheese. The limiting reactant is the cheese because some bread is left unused. The second example is assembling automobiles. Each auto requires one steering wheel, four tires, and other components. The limiting reactant is the tires, since they will run out first.
Explain why it is impossible to have a theoretical yield of more than 100%
Since the theoretical yield represents the maximum amount of product that can be obtained by a reaction from given amounts of reactants under any conditions, in an actual experiment you can never obtain more than this amount.
How many grams of NH3 will have the same number of molecules as 15.0 g of C6H6?
a. 3.27 g NH3
Which of the following has the largest number of molecules?
a. 1 g of benzene C6H6
b. 1 g of formaldehyde, CH2O
c. 1 g of TNT, C7H5N3O6
d. 1 g of naphthalene, C10H8
e. 1 g of glucose C6H12O6
b. 1 g of formaldehyde CH2O
How many atoms are present in 123 g of magnesium cyanide?
a. 9.7 x10²³
b. 2.91 x10²?
c. 2.83 x10²?
d. 4.85 x10²?
e. 5.65 x10²?
d. 4.85 x10²?atoms
When 2.56 g of a compound containing only carbon, hydrogen, and oxygen is burned completely, 3.84 g of CO2 and 1.05 g of H2O are produced. What is the empirical formula of the compound?
CHAPTER 4: Explain why some electrolyte solutions are strongly conducting, whereas others are weakly conducting.
Some electrolyte solutions are strongly conducting because they are almost completely ionized, and others are weakly conducting because they are weakly ionized. The former solutions will have many more ions to conduct electricity than will the latter solutions if both are present at the same concentrations.
Define the terms strong electrolyte and weak electrolyte. Give examples
A strong electrolyte is an electrolyte that exists in solution almost entirely as ions. An example is NaCl. When NaCl dissolves in water, it dissolves almost completely to give Na+ and Cl- ions. A weak electrolyte is an electrolyte that dissolves in water to give a relatively small percentage of ions. An example is NH3. When NH3 dissolves in water, it reacts very little with the water, so the level of NH3 is relatively high, and the level of the NH4+ and OH- is relatively low.
Explain the terms soluble and insoluble. Use the solubitility rules to write the formula of an insoluble ionic compound.
Soluble means the ability of a substance to dissolve in water. A compound is insoluble if it does not dissolve appreciably in water. An example of a soluble ionic compound is sodium chloride NaCl, and an example of an insoluble compound is calcium carbonate, CaCO3
What are the advantages and disadvantages of using a molecular equation to represent an ionic reaction?
The advantage of using a molecular equation to represent an ionic equation is that it states explicitly what chemical species have been added and what chemical species are obtained as products. It also makes stoichiometric calculations easy to perform. The disadvantages are:
1. the molecular equation does not represent the fact that the reaction actually involves ions.
2. the molecular equation does not indicate which species exist as ions and which exist as molecular solids or molecular gases.
What is a spectator ion? Illustrate with a complete ionic reaction.
A spectator ion is an ion that does not take part in the reaction. In the following ionic reaction, the Na+ and Cl- are spectator ions.
Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq) ? Na+(aq) + Cl-(aq) + H2O(l)
What is a net ionic equation? What is the value in using a net ionic equation? examples.
A net ionic equation is an ionic equation from which spectator ions have been canceled. The value of such an equation is that it shows the reaction that actually occurs at the ionic level. An example is the ionic equation representing the reaction of calcium (CaCl2) with potassium carbonate (K2CO3).
CaCl2(aq) + K2CO3(aq) ? CaCO3(s) + 2KCl(aq):
Ca²?(aq) + CO3²?(aq) ?CaCO3(s) (net)
What are the major types of chemical reactions? Give a brief description and an example of each.
3 major reactions are: precipitation reactions, acid-base reactions, and oxidation-reduction reactions.
(oxidation-reduction are: combination reactions, decomposition, displacement & combustion)
-Precipitation reaction: involves the formation of an insoluble solid compound.
2KCl(aq) + Pb(NO3)2(aq) ? 2KNO3(aq) + Pb2(s)
-Acid-base reaction (neutralization): results in an ionic compound and possibly water.
HCl(aq) + NaOH(aq) ? NaCl(aq) + H2O(l)
-combination reaction: 2 substances combine to form a third substance.
2Na(s) + Cl2(g) ? 2NaCl(s)
-decomposition reaction: in which a single compound reacts to give 2 or more substances
2HgO(s) ? 2Hg(l) + O2(g)
-displacement reaction: or single replacement: an element reacts w/ a compound displacing an element from it.
Cu(s) + 2AgNO3(aq) ? 2Ag(s) + Cu(NO3)2(aq).
-combustion reaction: reaction with oxygen, usually rapid release of heat to produce a flame. The products include one or more oxides.
CH4(g) + 2O2(g) ? CO2(g) + 2H2O (l)
Describe in words how you would prepare pure crystalline AgCl and NaNO3 from solid AgNO3 and solid NaCl.
To prepare crystalline AgCl and NaNO3, first make solutions of AgNO3 and NaCl by weighing equivalent molar amounts of both solid compounds. Then mix the two solutions together, forming a precipitate of silver chloride and a solution of soluble sodium nitrate. Filter off the silver chloride and wash it with water to remove the sodium nitrate solution. Then allow it to dry to obtain pure crystalline silver chloride. Finally, take the filtrate containing the sodium nitrate and evaporate it, leaving pure crystalline sodium nitrate.
Give an example of a neutralization reaction. Label the acid, base and salt.
An example of a neutralization reaction is:
HBr (acid) + KOH (Base) ? KBr (salt) + H2O(l)
Give an example of a polyprotic acid and write equations for the successive neutralizations of the acidic hydrogen atoms of the acid molecule to produce a series of salts.
An example of a polyprotic acid is carbonic acid, H2CO3. The successive neutralization is given by the following molecular equations:
H2CO3 (aq) + NaOH(aq) ? NaHCO3(aq) + H2O(l)
NaHCO3(aq) + NaOH(aq) ? Na2CO3(aq) + H2O(l)
Why must oxidation and reduction occur together in a reaction?
Since an oxidation-reduction reaction is an electron transfer reaction, one substance must lose the electrons and be oxidized while another substance must gain electrons and be reduced.
Give an example of a displacement reaction. What is the oxidizing agent? What is the reducing agent?
A displacement reaction is an oxidation-reduction reaction in which a free element reacts with a compound, displacing an element from it.
Cu(s) + 2AgNO3(aq) ? 2Ag(s) + Cu(NO3)2(aq)
Ag+ is the oxidizing agent, and Cu is the reducing agent
Why is the product of molar concentration and volume constant for a dilution problem?
The number of moles present does not change when the solution is diluted.
Describe how the amount of sodium hydroxide in a mixture can be determined by titration with hydrochloric acid of known molarity.
The reaction is:
HCl + NaOH ? NaCl + H2O
After titration, the volume of hydrochloric acid is converted to moles of HCl using the molarity. Since the stoichiometry of the reaction is 1 mol HCl to 1 mol NaOH, these quantities are equal.
Moles HCl = moles NaOH = molarity x volume
You could then multiply by the molar mass of NaOH to obtain the amount in the mixture.
What is the net ionic equation for the following molecular equation?
HF(aq) + KOH(aq) ? KF(aq) + H2O(l)
Hydrofluoric acid, HF is a molecular substance and weak electrolyte.
a. H+ (aq) + OH-(aq) ? H2O(l)
b. H+ (aq) + KOH(aq) ? K+ (aq) + H2O(l)
c. HF(aq) + KOH(aq) ? K+(aq) + F-(aq)
d. HF(aq) + K+(aq) + OH-(aq) ? KF(aq) + H2O(l)
e. HF(aq) + OH-(aq) ? F-(aq) + H2O(l)
An aqueous sodium hydroxide solution mixed with an aqueous magnesium nitrate solution yields which of the following products?
a. magnesium hydroxide (aq)
b. magnesium dihydroxide (s)
c. magnesium hydroxide (s)
d. dimagnesium hydroxide (s)
e. sodium nitrate (l)
c. Magnesium hydroxide(s)
Which of the following compounds would produce the highest concentration of Cl? ions when 0.10 mol of each is placed in separate beakers containing equal volumes of water?
In an aqueous 0.10 M HNO2 solution (HNO2 is a weak electrolyte), which of the following would you expect to see in the highest concentration?
What is the molar mass of the solid c6h8N4O4?
a. 192 g/mol
b. 144.4 g/mol
c. 200. g/mol
d. 136 g/mol
e. 104 g/mol
c. 200. g/mol
Consider the reaction of sulfur dioxide and water, which is represented by the following equation: 2SO2(g) + 2H2O(g) ? 2H2S(g) + 3O2(g)
How many moles of electrons are transferred according to the equation that is written?
Which of the following compounds is insoluble in water?
The concentration of Pb²? in a sample of wastewater is to be determined by using gravimetric analysis. To a 100.0mL sample of wasterwater is added an excess of sodium carbonate, forming insoluble lead (II) carbonate (267.20 g/mol) according to the balanced equation given below. The solid lead (II) carbonate is dried, and its mass is measured to be 0.1209 g. What was the concentration of Pb²? in the orginal wastewater sample?
Pb²?(aq) + Na2CO3(aq) ? PbCO3(s) + 2Na?(aq)
a. 0.0004525 M
b. 0.3231 M
c. 221.0 M
d. 0.001209 M
e. 0.004525 M
A molecular compound contains 92.3% carbon and 7.7% hydrogen by mass. If 0.572 mol of the compound weighs 29.74 g, what is its molecular formula?
Carbon = 92.3 /12(amu) Hydrogen = 7.7/1 (amu)
C = 7.69, H= 7.7
To find molecular weight take compound weight 29.74 g and divide by the moles 0.572 moles = 51.99 g/mol
MW = 52
EW = 13 (amu of both elements)
MW/EW = 4 molecular formula is C4H4
What is the net ionic equation for the neutralization of sulfuric acid with potassium hydroxide?
a. H2SO4(aq) + 2KOH(aq) ? 2H2O(l) + K2SO4(aq)
b. H2SO4(aq) + 2OH-(aq) ? 2H2O(l) + SO4²?(aq)
c. H2S(aq) + 2KOH(aq) ? 2H2O(l) + K2S(aq)
d. H+(aq) + OH-(aq) ?H2O(l)
e. 2H+(aq) + 2KOH(aq) ? 2H2O(l) + 2K+(aq)
d. H+(aq) + OH-(aq) ? H2O(l)
What is the percentage by mass of nitrogen atoms in ammonium sulfate, (NH4)2SO4?
Of the following, the only empirical formula is:
Which net ionic equation best represents the reaction that occurs when aqueous solution of ammonium carbonate is mixed with an aqueous solution of magnesium acetate?
a. (NH4)2CO3(aq) + Mg²?(aq) ? MgCO3(s) + 2NH4+(aq)
b. 2NH4+(aq) + Mg(C2H3O2)2(aq) ? 2NH4C2H3O2(aq) + Mg²?(aq)
c. 2NH4+(aq) + 2C2H3O2-(aq) ? 2NH4C2H3O2(aq)
d. (NH4)2CO3(aq) + Mg(C2H3O2)2(aq) ? MgCO3(s) + 2NH4C2H3O2(aq)
e. Mg²?(aq) + CO3²?(aq) ? MgCO3(s)
e. Mg²?(aq) + CO3²?(aq) ? MgCO3(s)
Which of the following is a strong acid in aqueous solution?
Which of the following do you need to know to be able to calculate the molarity of a salt solution?
I. the mass of salt added
II. the molar mass of the salt
III. the volume of water added
IV. the total volume of the solution
I, II and IV only
The mass of salt added, molar mass of salt and the total volume of the solution.
What volume of 0.54 M NaCl may be prepared by dilution of 0.100 L of a 6.0 M NaCl solution?
a. 0.90 L
b. 0.54 L
c. 1.9 L
d. 1.1 L
e. 0.91 L
d. 1.1 L
Which of the following is a strong electrolyte in aqueous solution?
The oxidation number of carbon in the formate ion, HCOO-, is:
Which one of the following is necessary in order for a metal to be oxidized?
a. removal of electrons
b. addition of oxygen
c. removal of oxygen
d. addition of hydrogen
e. addition of electrons
a. removal of electrons
What is the net ionic equation for the acid-base reaction that occurs when acetic acid and sodium hydroxide solutions are mixed?
a. HC2H3O2(aq) + NaOH-(aq) ? Na C2H3O2-(aq) + H2O(l)
b. HC2H3O2(aq) + OH-(aq) ? C2H3O2-(aq) + H2O(l)
c. H3O+(aq) + OH-(aq) ? 2H2O(l)
d. H+(aq) + OH-(aq) ? H2O(l)
e. C2H3O2-(aq) + H+(aq) + Na+(aq) + OH-(aq) ? Na+(aq) + C2H3O2-(aq) + H2O(l)
b. HC2H3O2(aq) + OH-(aq) ? C2H3O2-(aq) + H2O(l)