1. Topic 8 Acids and Alkalis

2. Starter 1.Name the oxide substance that can be used to form an acid. 2.a) What is produced at the negative electrode when an acid is electrolysed? b) How can you test for this? 3. What ions are present in alkalis? 4. What happens to the pH and concentration of hydrogen ions when an acid is diluted? 5. From which groups can soluble metal hydroxides be formed?

3. Calculations

4. Gram Formula Mass This is the total mass of a compound. It is the mass of one mole of a substance. Sometimes called Molar Mass. It takes into consideration the number and mass of each element in a compound.

5. Steps to Calculate Formula Mass 1.Correct Formula – Use valency rules! 2.Write down numbers of each elements in compound. 3.Add in mass of element – use pg4 in data book. 4.Add up total mass.

6. Example 1 Q. Work out the formula mass of sodium chloride 1. Work out Formula – Na Cl Cross over valencies– NaCl 2. Numbers of each element (1 x Na) + (1 x Cl) 3. Add in mass (1 x 23) + (1 x 35.5) 4. Add total mass 23 + 35.5 = 58.5 amu.

7. Example 2 Q. Work out the formula mass of Calcium hydroxide. 1. Work out Formula – Ca OH Cross over valencies– Ca(OH) 2 2. Numbers of each element (1 x Ca) + (2 x O) + (2 x H) 3. Add in mass (1 x 40) + (2 x 16) + (2 x 1) 4. Add total mass 40 + 32 + 2 = 74 amu.

8. Example 3 Q. Work out the formula mass of magnesium phosphate 1. Work out Formula – Mg PO 4 Cross over valencies– Mg 3 (PO 4 ) 2 2. Numbers of each element (3 x Mg) + (2 x P) + (8 x O) 3. Add in mass (3 x 24.5) + (2 x 31) + (8 x 16) 4. Add total mass 73.5 + 62 + 128 = 263.5 amu.

9. The Mole m = mass n = number of moles FM = Formula Mass m n FM Knowing any 2 of these factors allows you to calculate the third unknown: m = n x FM n= mFM = m FM n

10. Example 1 Q. Calculate the mass of 2 Moles of water 1.Work out FM for 1 mole a)Formula – H 2 O b)Numbers of elements – (2 x H) + (1 x O) c)Add in Mass - (2x1) + (1x 16) d)Formula Mass – (2x1) + 16 = 18 g 2.Use triangle m n FM m= n x FM m = 2 x 18 = 36g

11. Example 2 Q. Calculate the number of moles of 11.7g of sodium chloride. 1.Work out FM for 1 mole a)Formula – NaCl b)Numbers of elements – (1 x Na) + (1 xCl) c)Add in Mass - (1x23) + (1x35.5) d)Formula Mass – (23) + 35.5 = 58.5 g 2.Use triangle m n FM n= m = 11.7 = 0.2 moles FM 58.5

12. Example 3 Q. Calculate the number of moles 2.6g of calcium fluoride. 1.Work out FM for 1 mole a)Formula – CaF 2 b)Numbers of elements = (1 xCa) + (2 xF) c)Add in Mass = (1x40) + (2x19) d)Formula Mass = 40 + 38 = 78 g 2.Use triangle m n FM n= m = 2.6= 0.033 moles FM 78