The molar mass of water (H2O) is 18.02 g/mol. Yun needs 0.025 mol H2O for a laboratory experiment. She calculates that she needs 72.0 g H2O for the experiment. Is she correct?

The relation between moles and mass is given by the formula:

# of moles = (mass in grams) / (molar mass)

So, given the molar mass of 18.02 and 0.025 mol, you can solve the formula for the mass of water in grams:

mass of water in grams = # of moles of water * molar mass of water

mass of water in grams = 0.025 mol * 18.02 g / mol = 0.4505 g of water.

Then, she is wrong.