Empirical Formulas & Percent Composition CO 2 Carbon dioxide BCl 3 boron trichloride

Molar Mass

How many moles of alcohol are there in a “standard” can of beer if there are 21.3 g of C 2 H 6 O? (a)Molar mass of C 2 H 6 O = g/mol (b)Calc. moles of alcohol

How many molecules of alcohol are there in a “standard” can of beer if there are 21.3 g of C 2 H 6 O? = 2.78 x molecules We know there are mol of C 2 H 6 O.

How many atoms of C are there in a “standard” can of beer if there are 21.3 g of C 2 H 6 O? = 5.57 x C atoms There are 2.78 x molecules. Each molecule contains 2 C atoms. Therefore, the number of C atoms is

Empirical & Molecular Formulas A pure compound always consists of the same elements combined in the same proportions by weight. Therefore, we can express molecular composition as PERCENT BY WEIGHT Ethanol, C 2 H 6 O 52.13% C 13.15% H 34.72% O

Percent Composition Consider some of the family of nitrogen- oxygen compounds: NO 2, nitrogen dioxide Structure of NO 2 What is the weight percent of N and of O?

Percent Composition Consider NO 2, Molar mass = Percent of N and of O? What are percentages of N and O in NO? 46 g/ mol

Determining Formulas In chemical analysis we determine the % by weight of each element in a given amount of pure compound and derive the EMPIRICAL or SIMPLEST formula. PROBLEM : What is its empirical formula of a compound of B and H if 81.10% is boron?

B %B + %H = 100% 100% % B = 18.90% H. If there were g of the compound: there are g of B and g of H. Calculate the number of MOLES of each. Determination of empirical formulas: mass of each

Calculate the number of moles of each element assuming g total. Determination of empirical formulas: moles of each Determination of empirical formulas: moles of each

Take the ratio of moles of B and H. atoms combine in small whole number ratios Determination of empirical formulas: moles ratio Always divide by the smaller number mol H and mol B mol 2.5 mol H to 1.0 mol B

. But we need a whole number ratio. 2.5 mol H or 5 mol H 1.0 mol B 2 mol B EMPIRICAL FORMULA = B 2 H 5

If the empirical formula is B 2 H 5, what is its molecular formula ? We need to do an EXPERIMENT to find the MOLAR MASS. MS gives 53.3 g/mol Compare empirical mass = g/unit Find the ratio of these masses. Molecular formula = B 4 H 10

Determine the formula of a compound of Sn and I using the following data. Reaction of Sn and I 2 is done using excess Sn. Mass of Sn in the beginning = g Mass of Sn remaining = g Mass of iodine (I 2 ) used = g

Find the mass of Sn that combined with g I 2. Mass of Sn initially = g Mass of Sn recovered = g Mass of Sn used = g Find moles of Sn used: Tin and Iodine Compound

Now find the number of moles of I 2 that combined with 3.83 x mol Sn. Mass of I 2 used was g. ? How many mol of iodine atoms ? = x mol I atoms

Tin and Iodine Compound Now find the ratio of number of moles of moles of I and Sn that combined. Empirical formula is SnI 4

Practice Formula Determination Homework: Worksheets pgs Pg good review for mole calculations Prelab questions