Question

There is 2.26g of compound PX3 containing .8g P. What is the molar mass of element X given P = 30g/mol?

Answer 1

since the entire compound weighs 2.26g, and 0.8g of it must be phosphorus, the remaining mass must be element X: 1.46g
The formula is in a 1:3 ratio, but it's not a ratio of masses, it's a ratio of MOLES. Find the moles of P using the molar mass (30.99g/mol), and the number of moles of X must be three times that much. The mass of X (1.46g) divided by the moles of X gets a molar mass of about 18.9g/mol, or fluorine