1. Understanding Chemical Reactions Lesson: Calculations in Chemistry

2. The Mole

3. The mole The number of atoms in 1 gram of hydrogen is called a mole. 1 gram of hydrogen contains approx. 6 X 10 23 atoms. 600, 000, 000, 000, 000, 000, 000, 000 atoms – rather a lot!

4. The mole This number is called Avogadro’s constant.

5. The mole One mole of any substance always contains the same number of particles. The mass of one mole of a substance is called the Relative Atomic Mass (RAM).

6. R.A.M.

7. There are huge numbers of atoms even in test-tube reactions. So it is easier to talk about moles than the actual numbers involved.

8. Learn this equation!!!! Moles of atoms = mass R.A.M.

9. It’s like cashing in 1p coins at the bank. If you take a thousand 1 p coins to your bank, does the cashier count out each coin? The coins are weighed out on scales. The scales ‘know’ the mass of one hundred 1p coins, and tell the cashier how many pounds (£s) are on the scales.

10. Example How many moles of atoms are there in 2.4 g of carbon? Moles of atoms = mass / RAM = 2.4/12 = 0.2 moles

11. Practice Try these yourself! How many moles are in: 1.2g of hydrogen2. 36g of carbon 3. 160g of oxygen 4. 1.4g of nitrogen 5. 0.19g of flourine? (R.A.M.s H = 1, C = 12, O = 16, N = 14, F = 19)

12. Answers 1.2 moles2. 3 moles 3. 10 moles4. 0.1 moles 5. 0.01 moles

13. Relative Formula Mass Once you have a list of relative atomic masses, (which are shown on the Periodic Table), you can work out the relative mass of any molecule. The relative atomic of CO 2 The RAM of C is 12 The RAM of O is 16

14. Relative Formula Mass So for CO 2 we have: 1 carbon = 1 X 12 = 12 2 oxygen = 2 X 16 = +32 Therefore the relative formula mass of carbon dioxide is 44.

15. Example What is the relative formula (or molecular mass) of aluminium sulphate, Al 2 (SO 4 ) 3 (R.A.M.s: Al = 27, S =32, O =16)

16. Example What is the relative formula (or molecular mass) of aluminium sulphate, Al 2 (SO 4 ) 3 (R.A.M.s: Al = 27, S =32, O =16) 342 g !

17. Questions Answer the questions off p175 of the Chemistry text.