11.3 Moles of Compounds

Objectives: Recognize the mole relationships shown by a

chemical formula Calculate the molar mass of a compound Calculate the number of moles of a compound

from a given mass of the compound, and the mass of a compound from a given number of moles of the compound

Determine the number of atoms or ions in a mass of a compound

Enough about atoms:What about compounds?

The chemical formula for a compound tells us the types of elements and the number of each element contained in one unit of the compound.

Examples:

Ammonia (NH3) - 1 molecule contains:

1 atom of nitrogen and 3 atoms of hydrogen Baking soda (sodium hydrogen carbonate,

NaHCO3) – 1 formula unit contains:

1 atom of sodium, 1 atom of hydrogen, 1 atom of carbon, and 3 atoms of oxygen

Let’s take it a step further:

What if you had a mole of compounds? A mole of a compound contains as many

moles of each element as are indicated by the subscripts in the formula for the compound

For example, a mole of ammonia (NH3) consists of one mole of nitrogen atoms and three moles of hydrogen atoms.

Using the subscripts as conversion factors: Ex: NH3

1mol NH3 1 mol NH3

1 mol N atoms 3 mol H atoms

1 mol N atoms 3 mol H atoms

1 mol NH3 1 mol NH3

Example Problem:

1. Calculate the number of moles of hydrogen found in 3.50 moles of NH3.

K: UK:

10.5 mol Hydrogen

Example Problem:

2. Calculate the number of moles of carbon found in 9.85 moles of C6H12O6.

K: UK:.

59.1 mol carbon

The Molar Mass of Compounds

Summary of Getting the Molar Mass of Compounds:

The mass of a mole of a compound equals the sum of the masses of every particle that makes up the compound.

Use the formula to tell you how many of each element that is in the compound

Use the periodic table to get the masses of each element

Add them all up and you get the molar mass of the compound in units of g/mole

Example Problems of Molar Masses:

1) What is the molar mass of NH3?

17.04 g/mol 2) What is the molar mass of Sr(NO3)2?

211.64 g/mol

Using Molar Mass of Compounds

The molar mass of compounds can be used as a conversion factor, between mass and moles, just like we did with the molar mass of elements.

The only thing different is that you will have to calculate the molar mass of the compound before you can do any conversions using the molar mass.

Example Problems of Mole-Mass Conversions:

1) How many moles is 4.56 g of CO2 ?

K: UK:

0.104 moles CO2

2) How many moles is 46.8 g of CH4?

K: UK:

2.92 mol CH4

3) How many grams is 9.87 moles of H2O?

K: UK:

178g H2O

4) How many grams is 0.157 mol Fe2O3?

K: UK:

25.0 g Fe2O3

REVIEW:

What types of particles are contained in covalent compounds?

What types of particles are contained in ionic compounds?

Converting between the Mass of a Compound and the Number of Particles

Again this is the same process we followed for elements but this time we need to take into account what particle we are looking for in the problem.

Remember: The mole is the key There is no direct conversion between mass

and number of particles

Example Problems of Mass-Particle Conversions with Compounds

1) How many molecules in 6.8 g of CH4?

2a) How many formula units are there in 4.9 g of NaNO3?

2b) How many ions if the compound is made of Na+ and NO3

- ions?