1 mole of a pure substance has a mass equal to its molecular mass expressed in grams.

  This is known as the molar mass, M, and has the units g/mol

      So 2 moles of a substance would have a mass = 2 x molar mass

      3 moles of a substance would have a mass = 3 x molar mass etc

This leads to the formula: mass = moles x molar mass

  If we let:

      m = mass of substance in grams,

      n = moles of pure substance,

      M = molar mass of the pure substance in g/mol

 we can write the equation: m = n x M

This equation can be rearranged to give the following:

      n = m ÷ M   (moles = mass ÷ molar mass)

      M = m ÷ n   (molar mass = mass ÷ moles)

Calculate the mass of 0.25 moles of water

Write the equation:

m (in grams) = n x M

Extract the data from the question:

n = 0.25 mol

Calculate the molar mass of the substance using the periodic table:

M(H2O) = (2 x 1.008) + 16.00 = 18.016 g/mol

Substitute the values into the equation and solve:

mass = 0.25 x 18.016 = 4.504 g