Calculating Mass in Reactions

• 'Calculate the mass of sodium oxide (Na2O) that can be made by completely burning 5.0g of sodium (Na) in oxygen. Give your answer to 3 significant figures.'

  When you see a question like this, think of there being a known substance and an unknown substance:

  • The known substance is the one they've told you the mass of - in this case the sodium.
  • The unknown substance is the one you're trying to find the mass of - in this case the sodium oxide.
• There are 4 steps to follow for questions like this:

  1Write out the balanced equation

  2Find the moles of the known substance (moles = mass ÷ Mr)

  3Find the moles of the unknown substance (using the molar ratio from the balanced equation)

  4Find the mass of the unknown substance (mass = moles x Mr)