1. Honors Chemistry Section 7.3

2. A chemical formula indicates: ◦the elements present in a compound ◦the relative number of atoms or ions of each element present in a compound Chemical formulas also allow chemists to calculate a number of other characteristic values for a compound: ◦formula mass ◦molar mass ◦percentage composition

3. Formula Masses The formula mass of any molecule, formula unit, or ion is the sum of the average atomic masses of all atoms represented in its formula. ◦example:formula mass of water, H 2 O average atomic mass of H: 1.01 amu average atomic mass of O: 16.00

4. Problem Find the formula mass of Na 2 SO 3

5. Formula Masses The mass of a water molecule can be referred to as a molecular mass. The mass of one formula unit of an ionic compound, such as NaCl, is not a molecular mass. The mass of any unit represented by a chemical formula (H 2 O, NaCl) can be referred to as the formula mass.

6. Formula Masses Video

7. Problem Find the formula mass of potassium chlorate, HClO 3.

8. The Mole Video

9. Molar Masses The molar mass of a substance is equal to the mass in grams of one mole, or 6.022 × 10 23 particles, of the substance. ◦example: the molar mass of pure calcium, Ca, is 40.08 g/mol because one mole of calcium atoms has a mass of 40.08 g. The molar mass of a compound is calculated by adding the masses of the elements present in a mole of the molecules or formula units that make up the compound.

10. Calculating Molar Masses for Ionic Compounds

11. Problem What is the molar mass of barium nitrate, Ba(NO 3 ) 2 ?

12. Problem How many moles of each atom are in one mole of K 2 SO 4 ? How many moles of each atom are in one mole of (NH 4 ) 2 CrO 4 ?

13. Molar Mass as a Conversion Factor The molar mass of a compound can be used as a conversion factor to relate an amount in moles to a mass in grams for a given substance. To convert moles to grams, multiply the amount in moles by the molar mass: Amount in moles × molar mass (g/mol) = mass in grams

14. Calculations

15. Molar Mass as a Conversion Factor Video

16. Problem Calculate the molar mass of O 2. Use the molar mass of O 2 to convert 2.50 moles to mass

17. Problem What is the mass of 3.04 moles of NH 3 ?

18. Problem How many moles in 4.15 x 10 -3 g of C 6 H 12 O 6 ?

19. Problem How many molecules are in this mass of C 6 H 12 O 6 ?

20. Percentage Composition It is often useful to know the percentage by mass of a particular element in a chemical compound. To find the mass percentage of an element in a compound, the following equation can be used. The mass percentage of an element in a compound is the same regardless of the sample’s size.

21. Percentage Composition The percentage of an element in a compound can be calculated by determining how many grams of the element are present in one mole of the compound. The percentage by mass of each element in a compound is known as the percentage composition of the compound.

22. Water of Hydration Some compounds have water molecules as part of their crystal structure. These substances are call hydrates. ◦Ex. – Na 2 SO 4 ･10H 2 O This water is included when calculating the molar mass. You can drive this water off by careful heating.

23. Problem What is the percentage of water in CaSO 4 ·2H 2 O?

24. Problem NH 4 NO 3 is 35.0% N. How much N is in 49.0g of the compound?

25. Percentage Composition Video

26. Percentage Composition Calculations

27. Problem Find the percentage composition of Ag 2 SO 4.