Question

Core Chemistry Skill: Using Molar Mass as a Conversion Factor

Learning Goal:

To convert between mass and number of moles.

The conversion factor between mass and number of moles is called molar mass. By consulting the periodic table , we can determine that the molar mass of NaCl is

23.0 g/mole+35.5 g/mole=58.5 g/mole

This number tells us that 1 mole of NaCl has a mass of 58.5 g . The conversion factors are

58.5 g1 mole

and

1 mole58.5 g

Here is an example of the conversion of 12.0 moles of NaCl to grams using the first conversion factor:

12.0 moles×58.5 g1 mole=702 g

Part A

What is the mass of 6.70 moles of magnesium chloride, MgCl2 ?

Express your answer with the appropriate units.

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Part B

How many moles of nitrogen, N , are in 81.0 g of nitrous oxide, N2O ?

Express your answer with the appropriate units.

mass of MgCl2 =

Answer 1

A) Let us calculate molar mass og MgCl2.

Atomic mass of Mg = 24 g

Atomic mass of Cl = 35.5 g

Hence molar mass of MgCl2 = 24 + (2 x 35.5) = 95 g/mol

That means 1 mol of MgCl2 = 95 g

hence 6.70 moles of MgCl2 = 6.7 moles x 95 g/mol

= 636.5 g

Mass 6.70 moles of MgCl2 is 636.5 gm.

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B) Let us calculate molar mass of N2O.

At. Mass of N = 14

At.mass of O = 16

so, Molar mass of N2O = 2 x 14 + 16 = 44 g/mol.

That means 44 g of N2O = 1 mol of N2O

so, 81.0 g of N2O = 81.0/ 44 = 1.841 moles of N2O.

Now we can see that 1 mol of N2O = 2 moles of N atoms

So, 1.841 moles of N2O = 2 x 1.841 = 3.682 moles of N atoms.

Hence, 81 g of N2O = 1.841 moles of N2O = 3.682 moles of N atoms.