I need help with this stoichiometry on my homework

Question

In the reaction below, how many molecules of PCl₅ are required to react completely with 18 grams of water?

PCl5 + 4H2O → 5HCl + H₃PO₄

A. 1.5 x 10²³

B. 3.0 x 10²³

C. 6.0 x 10²³

D. 2.4 x 10²⁴

I keep getting option D and I can't find where I'm going wrong.

Answer 1

Use factor labeling ladder (This is a foundational problem and must be mastered. Start with how much stuff given (extensive variable)

18 g W * (mole/g W) * ( mole PCl/mole W) * (molecules/mole PCl) = molecules PCl

Cancel units (multiplication) until you get what you need

Now fill in conversions and quantitative physical relationships (Basically anything that has a "per" in it or any equality that applies)

1^st is molar mass of water which is 18 g/ 1 mole (you put the numbers in the right place above (i.e. divide by 18, but what you are doing is multiplying by one water mole per 18 grams)

2^nd moles PCl₅/mole W comes from the stoichiometry

3^rd N_A is always the number of atoms, molecules, ions per mole.