According to the left side of the chemical equation provided, you need 2 units of liquid water (H2O(l)) to react with each unit of solid calcium (Ca(s)). So, for 3.5 moles of Ca you will need 2 x 3.5 = 7 moles of water.
Now you need to convert moles of water to grams. Use the periodic table to find the grams per mole of H2O:
H = 1 g/ mole
O = 16 g/ mole
In water, there are 2 H atoms and 1 O atom. Therefore, 1 mole of H2O is 2+16 = 18 g / mole.
Remember, we need 7 moles of water to react with 3.5 moles of Ca.
7 moles x 18 g/mole = 126 g.
The answer is 126 g of water.
Note: The density of water is about 1 g/mL, so 126 g is about 126 mL (a "cup" of liquid in the US is about 160 mL, so this is about 3/4 of a cup). The mass of Ca is 40 g/ mole, so 3.5 moles is 140 g. A cup of sugar weighs about 200 g, so this would be about 3/4 cup of Ca powder (or pellets).
