1) First, let's find out how many moles of O are in 1 mol of Ca(NO₃)₂
It has:
1 mol of Ca
If the compound is enclosed in parentheses, you must multiply the value outside the parentheses by each atom of each element that is within the parentheses. In this case the value is 2, so it is necessary to multiply 2x1 (because inside the parentheses there is an atom of N) and multiply 2x3 to find the amount of oxygen (because inside the parentheses there are 3 atoms of O)
2 mol of N
6 mol of O
2) Now let's use the rule of 3 to find out how many moles of O are in 1.9 mol of Ca(NO₃)₂
1.9 mol of Ca(NO₃)₂ ------ x mol of O
1 mol of Ca(NO₃)₂ -------- 6 mol of O
x = 1.9 × 6
x = 11.4 moles of O
Answer: 11.4 mol O