Hello, Ur,
The first thing to notice is that the equation is not balanced. The are 2 chlorine atoms in the products but only 1 in the reactants. Same for the hydrogens. Balance both of them by by adding a "2" in front of the HCl:
Ca + 2HCl → CaCl2 + H2
This tells us we need two moles of HCl for every one mole of Ca, a molar ratio of 2 moles HCl/mole Ca.
Now let's calculate the number of moles of Ca there are in 10 grams of Ca. Do that by dividing the mass by the molar mass of Ca.
Moles Ca = 0.249 moles Ca
Now we can determine the moles HCl we need:
(0.249 moles Ca)*(2 moles HCl/1 mole Ca) = 0.499 moles HCl
Convert that to grams by multiplying moles HCl by it's molar mass (36.45). grams HCl to completely react (and no more) with C=10 grams of Ca = 18.2 grams HCl.
I hope this helps,
Bob
