# Chem 130 Ch3 – Flashcards

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molecular mass
the sum of the atomic masses of all the atoms in a molecule of the substance. Ex. Water is 2 +16=18 amu
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formula mass
the sum of the atomic masses of all the atoms in a formula unit of the compound salt(NaCl) is 23+35.5=58.5 amu
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mole (mol)
the quantity of a given substance that contains as many molecules or formula units as the number of atoms in 12g of carbon-12(6.02E23)
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6.02E23 Ex. a mole of ethanol equals 6.02E23 ethanol molecules
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molar mass
the mass of one mole of the substance. for all substances, the molar mass in g/mol is numerically equal to the formula mass in amu. Ex. the molar mass of C2H6O is 46 g/mol (because the amu is (12x2)+(1x6) +16= 46 amu)
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What do you use to convert from grams to moles?
molar mass in g/mol Ex. How many grams in .0654 moles of ZnI2? .0654mol(319 g ZnI2/1 mol ZnI2)=20.9 g ZnI2
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percentage composition
the MASS percentages of each element in the compound. Ex. 40% C 60% H
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mass percentage
mass% of element A=(mass of element A in the whole/ mass of the whole) x 100%
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How do you determine the empirical formula from masses of the elements?
Convert the masses to moles, then divide each by the smaller one to get whole integers. If you get 1.25, multiply both by 4 etc Ex. .483g N(1 mol N/14 g N)= .0345 mol N 1.104 g O (1 mol O/16 g O)= .069 mol O .345/.345= 1 N .069/.345= 2O NO2
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How do you determine the empirica formula from percentage compostions?
First, you assume you have 100 g of the substance, meaning that the percents will easily become grams. Then you follow the process as if they were masses Ex. 17.5%, so you assume 100g, so you'd have 17.5 g. Then convert all of the grams to moles then divide by the smallest etc
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How do you determine the molecular formula from empirical formula?
First, find the empirical formula. Then find that empirical formulas formula mass(by adding up amu's). Then, n=molecular mass/empirical mass. This n is the multiple to find your molecular mass. Ex. You find an empirical formula to be CH2O. You then find its empirical mass to be 30 amu (using amu's on the periodic table). Then divide the molecular mass(given in the problem as 60 amu, it will be given) by the empirical mass to get n. In this case it is 60/30=2. Then multiply this number to the empirical formula, so (CH2O)2= C2H4O2
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A 10.51 gram sample of an organic compound containing C, H and O is analyzed by combustion analysis and 21.01 grams of CO2 and 8.601 grams of H2O are produced. In a separate experiment, the molar mass is found to be 88.11 g/mol. Determine the empirical formula and the molecular formula of the organic compound.
First find moles of C and H, by making the grams of CO2 and H2O moles(by multyplying by the molar mass), then multiplying the mole to mole ratio (ex H2O to H would be (2/1) because there is 2 moles of H for every mole of H2O). Then, convert both moles of C and H to grams(using molar mass), then subtract these grams from the 10.51 sample to get grams of ) in the original sample. Then convert O grams to moles(molar mass), then divide the 3 moles by the smallest to get the integers. The answer for this question is C2H4O. To the molecular formula, divide 88.11/empirical formula mass, making the final answer c4h8o2
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Stoichiometry