Moles & Mass (grams)

Chemistry

Unit 6: Chemical Quantities

Lecture 6.2

Objectives

• Convert a quantity of a chemical between mass (in grams) and moles.

Background

The BEST way to bake bread:1 egg = 57 grams1 dozen = 12

Industrial recipe calls for 1,197 g of eggs. How many is this in dozens?

1.75 dozen

Why Convert Between Grams & Moles?

• Moles are a convenient unit because atoms are SOOOOOO SMALL!

• Chemical equations are written in terms of moles

• But we do not have a way to “measure” moles (‘cuz they’re different for every substance)

• BUT…we CAN measure grams! (on a balance!)

So How Do You Do It?

It’s just like any other conversion problem…you need given information and equalities from which to form conversion factors…then you just MULTIPLY!!

Can You Be More Specific?

1. Write down the given information (in grams or moles) and put it over “1”

2. Determine the molar mass of the substance (see Lecture 6.1)

3. Form a conversion factor from the molar mass equality (that’s just a fraction!)

4. Write a multiplication problem that cancels out the given units and let’s you end up with the wanted units

5. Plug ‘n’ Chug!

Let Me See An Example…

How many moles are in 47 g of NaCl?

47 g NaCl x 1 mol NaCl = 1 58.44 g NaCl

0.80 mol NaCl

Another Example

Find the mass of 3.6 mol of AgNO3

3.6 mol AgNO3 x 169.88 g AgNO3 =

1 1 mol AgNO3

611.57 g AgNO3

Practice

Determine the number of moles of each:

• 15.5 g SiO2

• 0.0688 g AgCl

Find the mass of each:

• 1.50 mol C5H12

• 14.4 mol F2

0.26 mol SiO2

0.00048 mol AgCl

(4.80 x 10-4 mol AgCl)

108.26 g C5H12

547.20 g F2

Quick Quiz1. Why do we need to be able to convert between

grams and moles?

2. Where do you get the needed equality to convert grams to moles OR moles to grams?

3. Explain how to set up a conversion problem to convert a number of moles of a substance to grams of that substance.

4. How many moles are in 450 g of HF?

5. Find the mass of 2.7 mol of H2O.

Chemical equations are written in terms of moles, but we cannot measure moles in the lab…we can only measure grams on a balance.

From the molar mass of that substance. You will need the chemical formula and a periodic table!

Put the given information over “1” then multiply by the molar mass conversion factor that has grams on top and moles on the bottom.

22.49 grams of HF

48.65 mol H2O