1. Ch 7 The Mole and Chemical Composition
What are some things that are sold by weight instead of by number?
Which would need a larger package, a kilogram of pencils of a kilogram of drinking straws?
If you counted one person per second, how many hours would it take to count the 6 billion people in the world?

2. Mole
Unit to measure amount of substance whose particles are e same as the number of atoms in exactly 12 g of carbon-12
Avogadro’s number x 1023
Counting unit for large number of particles too small too see

4. Molar mass Mass in grams of one mole of a compound or element
Relate moles to mass

5. Moles of an element Use moles to count by weight.
? Moles of carbon are in 26g
?= 26 g C x 1mol C = 26 molC =2.2
12.0g C mol
2.2molC x 6.02x1023atoms=1.3x1023
1molC C atoms

6. Compounds ? Molecules CO2 in 0.75moles
?=0.75mol CO2x 6.02x1023molecule
1mol CO2
= 4.5x1023molecules CO2

7. Relative Atomic Mass Isotopes have different masses
Isotopes have different numbers of neutrons
Periodic table – mass is average of all naturally occurring isotopes

8. Chemical Formulas and Moles
Formulas- what elements & how many of each
Covalent compounds – how many of each element
Ionic compounds – simplest ratio of cations to anions
Polyatomic ions – simplest ratio of cations to anions

9. Molar Mass of a compound
molar mass of elements multiplied by subscript
C12H22O11
C = 12.0g x 12 = 144.0g
H = 1.0g x 22 = g
O = 16.0g x 11 = 176.0g
total = g

10. Formulas and Percentage Composition
Analyze compounds to find percentage composition
Percent by mass of each element in a compound
Empirical formula- simplest ratio among the atoms found in a compound
C H3 = C2 H6
empirical molecular

12. Step one

13. Step 2

14. Step 3

15. Summary

16. Percentage Composition

17. Find mass of one element
Calculate the mass of zinc in a g sample of zinc nitrate, Zn(NO3)2.
Items
Data
Mass of zinc nitrate
30.00 g
Formula of zinc nitrate
Zn(NO3)2
Molar mass of zinc nitrate
g/mol
Mass of zinc in the sample
? g

18. Find % of Zn

20. Determining Empirical Formulas

21. Empirical Formulas
Determine the empirical formula for an unknown compound composed of
36.5% sodium, 38.1% oxygen, and 25.4% sulfur by mass.

22. Steps 1-4

23. Compute

24. Mole ratios
From the calculations, the simplest mole ratio is 2 mol Na :3 mol O: 1 mol S.
The simplest formula is therefore Na2O3S. Seeing the ratio 3 mol O : 1 mol S,

26. Molecular Formula
A compound is analyzed and found to have the empirical formula CH2O.
The molar mass of the compound is found to be 153 g/mol. What is the compound’s molecular formula?

27. Plan

28. Compute

29. Comparing Formulas