Question

A solid hydrate with a mass of 2.125 g was heated to drive off the water. A solid anhydrous residue remained, who’s mass was 1.044 g. Calculate the percent water in the hydrate. If the anhydrous residue has a formula weight of 104 g, how many water molecules are present in each molecule of anhydrous salt

Answer 1

mass of hydrate = 2.125
mass of the anhydrous residue = 1.044

the anhyrous residue is what's leftover after the water is driven off. therefore, mass of water = mass of hydrate - mass of anhydrous residue

the mass of the water, therefore, must be 2.125g - 1.044g

to calculate the % water in the hydrate, divide (anhydrous residue) / (hydrate) and multiply by 100%

now, we need to determine how many moles of anhydrous residue we have: divide mass of anhydrous residue / formula weight of anhydrous residue

similarly, we need to determine how many moles of water (mass of water / molar mass of water)

then we find the ratio of (moles water) / (moles residue) to find out how many water molecules are present per 1 molecule anhydrous salt