1. Math in Chemistry

2. Percent Composition Purpose: Can be used to figure out chemical formulas.

3. I. Percent Composition Two different types of problems: 1) Masses are given 2) No Masses are given

4. Masses are Given Steps to solve problem: 1) Add given masses to get total mass for one compound 2) Divide mass of each element by the total mass 3) Multiply by 100 to get the percent

5. Masses are Given Examples 1) A sample of Silver sulfide was found to contain 29.0 g of Ag and 4.30 g of S. Calculate the percent composition.

6. Masses are Given Examples 2) 222.6 g of Na combines completely with 77.4 g of O. Calculate the percent composition.

7. No Masses Given Steps to solve problem: 1) Assume you have 1 mole of the compound and calculate its molar mass 2) Determine the molar masses of each element in the compound 3) Divide the molar mass for the element by the molar mass of the compound 4) Multiply by 100 to get the percent

8. No Masses Given Examples 1) Calculate the percent composition of C and H in ethane, C 2 H 6.

9. No Masses Given Examples 2) Calculate the percent composition of sodium hydrogen sulfate.

10. II. Calculate the Mass of an Element in a Compound Steps to solve the problem: 1) Find the molar mass of the compound 2) Find the percent composition of the element 3) Set up a conversion factor problem Given mass (g compound) x percent composition/100 (g compound)

11. Calculate the Mass of an Element in a Compound Examples 1) Calculate the mass of hydrogen in the following: A) 350 g C 3 H 8 B) 20.2 g NaHCO 3 C) 378 g HCN

12. III. Empirical Formulas Empirical formulas are the lowest whole number ratio of the atoms of the elements in a compound. Molecular Formula C 2 O 4 Na 2 O 2 Empirical Formula CO 2 NaO

13. B. Empirical Formula C2H6C2H6 CH 3 reduce subscripts Smallest whole number ratio of atoms in a compound

14. B. Empirical Formula 1. Find mass (or %) of each element. 2. Find moles of each element. (divide given mass by molar mass) 3. Divide answers by the smallest # to find subscripts. 4. When necessary, multiply subscripts by 2, 3, or 4 to get whole #’s.

15. B. Empirical Formula Find the empirical formula for a sample of 25.9% N and 74.1% O. 25.9 g 1 mol 14.01 g = 1.85 mol N 74.1 g 1 mol 16.00 g = 4.63 mol O 1.85 mol = 1 N = 2.5 O

16. B. Empirical Formula N 1 O 2.5 Need to make the subscripts whole numbers  multiply by 2 N2O5N2O5

17. Empirical Formulas Examples 2) What is the empirical formula for a compound that is 79.8% C and 20.2% H?

18. Empirical Formulas Examples 3) What is the empirical formula of a compound that is 67.6% Hg, 10.8% S, and 21.6% O?

19. IV. Molecular Formulas Usually the empirical formula is the molecular formula for a compound. When it is not, the molecular formula is defined as the elements and number of atoms that are contained in a compound.

20. IV. Molecular Formulas Molecular formulas are always multiples of empirical formulas. CH 3 C 2 H 6

21. IV. Molecular Formulas Steps to solve the problem: 1) Determine the empirical formula 2) Divide the molecular mass by the empirical formula mass to get a ratio. 3) Multiply the elements’ subscripts by the number you get in step 2.

22. Molecular Formulas Examples 1) Calculate the molecular formulas of the following compounds: Molecular MassEmpirical Formula 60 gCH 4 N 78 gNaO 181.5 gC 2 HCl

23. Molecular Formulas Examples 2) The compound methyl butanoate smells like apples. Its percent composition is 58.8% C, 9.8% H, and 31.4% O. If the molecular mass is 102 g/mol, what is the molecular formula?

24. Molecular Formulas Examples 3) You find 7.36 g of a compound has decomposed to give 6.93 g of oxygen. The rest is hydrogen. If the molecular mass is 34.0 g/mol, what is the molecular formula?

25. STP STP = Standard temperature and pressure Where, Temperature = 0°C = 273 K Pressure = 101.3 kPa = 1 atmosphere (atm) For ANY gas at STP, 1 mol = volume of 22.4 L (molar volume) = 6.02 x 10 23 particles of that gas

26. Unit of density of a gas = g/L Can be used to calculate the gram formula mass of that gas Example 1: Determine the volume of 0.600 mol of sulfur dioxide gas at STP

27. More Examples Determine the # of moles in 52.5 L of He gas at STP Find the Number of Particles in 30 L of sulfur gas at STP Find the mass of 43 L of nitrogen gas at STP