The reaction between CaCO3 and HCl can be represented as -

2HCl   +   CaCO₃ → CaCl₂ +   H₂O

So, 1 mole of CaCO3 reacts with 2 moles of HCl.

CaCO3 mol wt = 100 g/mol

1 mole = 100 gm.

100 gm of CaCO3 will react with 2 moles of HCl.

1M HCl = 1 mole per litre.

1000 ml = 1 mole

100 ml = 0.1 mole

2 moles of HCl reacts with 1 mole of CaCO3

Then, 0.1 mole of HCl reacts with [1/2]*0.1

= 0.05 moles

1 mole of CaCO3 = 100 gm

So, 0.05 mole of CaCO3 = 5 gm.

Therefore, 5 gm of CaCO3 has reacted with HCl, and the unreacted amount is 100 – 5 = 95 gm.