± Conversions Involving Moles

Question

The mole is defined as the number of atoms in exactly 12g  of carbon-12. The mole equals 6.022*10^23 items of whatever you are counting. This number is also called Avogadro's number, after Amadeo Avogadro (1776–1856).

Molar mass is the mass of 1 mol of atoms of an element; for compounds, molar mass is the mass of 1 mol of molecules or formula units. Molar mass can be used to convert mass to moles for any element or compound.

Part A- How many grams are in 1.60 mol of sodium chloride (NaCI)

Part B- How many water molecules are in a block of ice containing 0.500 mol ofwater (H2O)

Answer 1

for part (a) you can just multiply by the molar mass of NaCl to convert it over

1.60 mol x 58.44 g/mol = 93.5 grams

then for part (b) you can multiply by Avogadro's number to get the molecules

0.500 mol x 6.022 x 10^23 molecules/mol = 3.011 x 10^23 molecules

Answer 2

We can solve these simply by using dimensional analysis and the correct conversion factors.

a). molar mass NaCl = 58.4 g / mol (add atomic mass Na and atomic mass Cl)

dimensional analysis:

1.60 mols NaCl x 1 mol NaCl / 58.4 g = 93.4 g NaCl (3 sig. figs.)

b). dimensional analysis:

0.500 mols H₂O x 6.022x10²³ molecules / mol = 3.01x10²³ molecules H₂O (3 sig. figs.)