Use Avogadro's number to find the number of oxalic acid molecules in 0.189 mol of H2C2O4.

1.138 * 10²³ molecules of oxalic acid

Explanation:

Avogadro's number is equal to 6.022 * 10²³ and it tells you the number of molecules in 1 mole of substance.

So now we devise the following reasoning:

if 1 mole of oxalic acid contains 6.022 * 10²³ molecules

then 0.189 moles of of oxalic acid contains X molecules

X = (0.189 * 6.022 * 10²³) / 1 = 1.138 * 10²³ molecules of oxalic acid