A 1.547 g sample of hydrate CuSo4 is heated to drive off the water. The hydrate crystals that are left behind have a mass of .989 g. Find X in the following formula

Question

CuSo4*XH2O Please help

Answer 1

Mass of water present = 1.547 g - 0.989 g = 0.558 g water

moles water = 0.558 g x 1mole/18 g = .0.031 moles H₂O

moles CuSO₄ = 0.989 g x 1 mole/160 g = 0.00618 moles

Dividing all by 0.00618 we obtain

moles CuSO₄ = 1

moles H₂O = 5

Therefore, X = 5

Answer 2

Hi! Let's see if we can break this calculation down for you.

1. A hydrate molecule has two parts - salt and water. You can add heat to get rid of the water, leaving the salt behind.
2. The mass of the hydrate that you start with (1.547 g) contains the mass of both the salt and the water. After heating, the water is evaporated off and the mass of what is left behind (0.989 g) is the mass of just the salt.
3. The difference between the starting mass (1.547 g) and the ending mass (0.989 g) is the mass of the water that evaporated off.
4. Now, we know the mass of the salt and the mass of the water in the hydrate sample. Next, we need to convert each of these masses to moles using molar mass (g/mol).
5. To find the molar mass for CuSO4, you are adding the mass of 1 Cu, 1 S and 4 O atoms and putting the unit g/mol on the end. Dividing the salt mass (0.989 g) by the molar mass will give you the moles of salt in the hydrate sample.
6. To find the molar mass for H2O, you are adding the mass of 2 H and 1 O atom and putting the unit g/mol on the end. Dividing the water mass (answer from #3) by the molar mass will give you the moles of water in the hydrate sample.
7. Dividing moles of water (answer to #6) by moles of salt (answer to #5) should give you a number pretty close to a whole number - that whole number is your value for X in the hydrate formula.

Hope this helps!