Stephen is investigating the rate of the reaction between calcium carbonate and
hydrochloric acid. The equation for the reaction is;

CaCO3 + 2HCl CaCl → 2 + H2O + CO2

He adds 5.0 g of calcium carbonate to an excess of hydrochloric acid and measures the mass of carbon dioxide given off every 10 seconds. The results of his reaction are shown in the table below.
Time in seconds Mass of CO2 produced in grams
0 0
10 13
20 18
30 22
40 24
50 25
60 26
70 26
80 26

a) Calculate the mean rate of the reaction after 15 seconds
b) After how many seconds has the reaction finished?