We have a sample of 5.3 g of Na₂CO₃. Before finding the number of molecules we have to find the number of moles. When we are given the mass of a sample and we have to find the number of moles in that sample, we usually use the molar mass. The molar mass can be calculated using the atomic masses of the elements that are in the compound. If we look fot the atomic masses, we find:
Na: 22.99 amu C: 12.01 amu O: 16.00 amu
Since one molecule of Na₂CO₃ has 2 atoms of Na, 1 atom of C and 3 atoms of O, the molar mass of Na₂CO₃ is:
molar mass of Na₂CO₃ = 2 *22.99 + 1 *12.01 + 3 *16.00
molar mass of Na₂CO₃ = 105.99 g/mol
Now that we found the molar mass of Na₂CO₃, we can find the number of moles that we have in the 5.3 g sample of it.
number of moles of Na₂CO₃ = 5.3 g / (105.99 g/mol)
number of moles of Na₂CO₃ = 0.050 moles
In one mol of molecules we have 6.022 *10^23 molecules. using that relationship we can find the answer to our problem:
1 mol of molecules = 6.022 *10^23 molecules
number of molecules = 0.050 moles * 6.022 * 10^23 molecules/1 mol
number of molecules = 3.0 * 10^23 molecules
Answer: there are 3.0 * 10^23 molecules in 5.3 g of sodium carbonate.