[shek]

I have a question on calculating the average rate of reaction.
Here is the question:
CaCO₃+2HCl  CaCl₂+CO₂+H₂O
When 2g of lumps of calcium carbonate was added to 50cm3  of 2M HCl, the CaCO₃ reacted and disappear in 120s.

What is the average rate of the whole reaction expressed in terms of rate of consumption of HCl in mol/dm/s?

solution is average rate of consumption =0.0167/(40.1+12+16x3)
=1.67x 10^-4
Average rate of consumption of HCl
=1.67x10^-4X2/0.05
=6.68x10^-3


but why does not it works?
rate of consumption of HCl
=0.01998(no of mol of CaCO₃)x2/0.05/120
 I just do it according to the required units in the quedtion but there is errors

Mod Edit: fixed right arrow

[Burner]

solution is average rate of consumption =0.0167/(40.1+12+16x3)
=1.67x 10^-4
Average rate of consumption of HCl
=1.67x10^-4X2/0.05
=6.68x10^-3


but why does not it works?
rate of consumption of HCl
=0.01998(no of mol of CaCO₃)x2/0.05/120
 I just do it according to the required units in the quedtion but there is errors

I got the same result using your two methods so I don't see any errors in your expression. However, I think that step-by-step calculations are better because they are easier to follow and check.