Calculating the number of particles in a given amount of substance

· We know how to work out the number of moles of any substance, right?

· We also know that 1 mole = 6 x 1023

· (and 2 moles = 1.2 x 1024, 0.1 moles = 6 x 1022 , etc) right?

We are going to use these facts to work out the number of particles (atoms, ions, molecules) in given amounts of different substances. This can get a tad tricky, so think carefully and set out your work clearly. And remember, working out the number of moles is the key to everything!

Look at the following formulae and the amounts of each substance given in Q1-10 below

1. Name each

2. Decide which is term (RAM / RMM / RFM) is appropriate and then calculate it

3. If necessary, work out the number of moles of the substance present from the data given (if not already given)

4. If the substance is ionic, work out the number of moles of each ion present, and then the total number of ions

OR

5. If the substance is covalent, work out the number of moles of each atom present, and then the total number of atoms.

1. NaCl (2 moles)

2. CO2 (O.5 moles)

3. Ca(OH)2 (148g)

4. C2H5OH (2.3g)

5. He (200 cm3 at room temperature and pressure)

6. Na2CO3 (50g)

7. Al2(SO4)3 (0.5 moles)

8. C2H4 (2.8g)

9. (NH4)2SO4 (2 moles)

10. 0.2 dm3 of 0.1 mol dm-3 Na2SO4 solution

11. Finally, go back and work out the number of molecules present in Q2,4 and 8

Ans to Q1-11 below

For some more practice: try Edexcel Chemistry for AS, page 17, Test Yourself Q18-22 (and especially try Q20-22).

· Make sure you self-mark using the answers on the website!