7.2: Molar Mass

Why can't we weigh a single atom or molecule on a balance?

A single atom or molecule is much too small to weigh, even on the most accurate balance.

What is the molar mass of an element?

The molar mass is the quantity in grams that equals the atomic mass of that element, representing 6.022 × 10²³ atoms.

How is the molar mass of carbon determined from the periodic table?

The atomic mass of carbon is 12.01, so 1 mol of carbon atoms has a mass of 12.01 g.

How much does 1 mol of silver atoms weigh?

107.9 g.

How much does 1 mol of sulfur atoms weigh?

32.07 g.

What are the steps to determine the molar mass of a compound?

1. Obtain the molar mass of each element.

2. Multiply each molar mass by the number of moles (subscript) in the formula.

3. Add the masses of the elements.

Calculate the molar mass for lithium carbonate (Li₂CO₃). Step 1: Obtain the molar mass of each element.

Li = 6.94 g/mol, C = 12.01 g/mol, O = 16.00 g/mol.

Calculate the molar mass for lithium carbonate (Li₂CO₃). Step 2: Multiply each molar mass by the number of moles in the formula.

• Li: 6.94 g/mol × 2 = 13.88 g/mol

• C: 12.01 g/mol × 1 = 12.01 g/mol

• O: 16.00 g/mol × 3 = 48.00 g/mol

Calculate the molar mass for lithium carbonate (Li₂CO₃). Step 3: Add the masses of the elements.

13.88 g/mol (Li) + 12.01 g/mol (C) + 48.00 g/mol (O) = 73.89 g/mol.

How do you use the periodic table to find the molar mass of an element?

Look at the atomic mass of the element on the periodic table.

Why do we round the molar mass of an element to the hundredths place or use at least four significant figures?

To ensure accuracy in calculations.