First, recall that one mole of CaCO₃ is 100 g because Ca=40, C=12, O=16×3. So 25 g is 0.25 moles. You must write the balanced equation, likely for thermal decomposition, and then use the mole ratio. My own struggle is that I practice these sums by candlelight after power cuts, and I have found that writing each atomic mass clearly at the top of the page prevents silly mistakes.
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