2Ca + O2 --> 2CaO
Consider the reaction below. How many grams of Ca are used in this reaction if 15.0g of CaO are produced (if O2 is present in excess)?
How many moles CaO is 15.0 grams? mols = grams/molar mass
Looking at the coefficients in the balanced equation, how many moles Ca are necessary to produce the moles CaO in 15.0 grams?
Convert mols Ca to grams Ca using the formula in line one of this post.
Post your work if you get stuck.