Agallon of water has a mass of 3.79 kg. how many moles of water (18.02 g/mol) is this?

First, we will convert the mass of the gallon to grams:
a gallon of water has a mass of 3.79 * 1000 = 3790 grams of water

number of moles can be calculated using the following rule:
number of moles = mass / molar mass
Therefore,
number of moles = 3790 / 18.02 = 210.32 moles

210. moles To determine the number of moles of a substance you have, simply divide the mass of the substance you have by its molar mass. So 3.79 kg / 18.02 g/mol = 3790 g / 18.02 g/mol = 210.3218646 mol Since we only have 3 significant figures, the answer should be rounded to 3 figures, giving 210. as the result. Notice that the decimal point is required. If you state the number of moles is 210, then that answer will only have 2 significant figures. But the value of 210. has 3 significant figures.