If liming a soil needs 100 g of calcium oxide, the mass of calcium carbonate doing the same work is about:

1. The two liming agents work through the same number of moles, so convert by moles. 2. Relative formula mass of $CaO$ is $40+16=56$; that of $CaCO_3$ is $40+12+48=100$. 3. Moles of $CaO$ used: $\dfrac{100}{56}=1.786$ mol. 4. The same number of moles of $CaCO_3$ has mass $1.786\times100=178.6$ g. 5. So about 179 g is needed — more than the oxide, because the carbonate carries extra mass that does no neutralising. _Source: NECTA ACSEE 2024 Chemistry 132/1, Question 8: Soil Chemistry and Environmental Chemistry_