What is the formula of the hydrate?

Question

A 15.67 g sample of a hydrate of magnesium carbonate was carefully heated, without decomposing the carbonate, to drive off the water. The mass was reduced to 7.58g.
Also, can you explain how you do it?

Answer 1

At the end you have pure MgCO3

MW = 84.32 g/mole

so 7.58 g = 7.58 g* mole/84.32g = 0.0898 moles

you hydrate it MgCO3*xH2O x = 1,2 3 or another integer, but the maximum amount of moles you will be able to have is 0.0898 mole

H2O = 18 g/mole

0.0898 moles (84.32g/mole+ x18 g/mole) = 15.67g

7.58 + 1.616x = 15.67 ==> x = 5

the hydrate is MgCO3•5H2O

Answer 2

A hydrate is a chemical that has water molecules loosely bonded to it. The water molecules are not actually part of the formula, so the formula is written slightly differently. An example would be CaSO4 . 3H2O. This chemical would be called calcium sulfate trihydrate.
IN THE PROBLEM PRESENTED BY YOU:
The mass is reduced due to loss of water,
MgCO3.nH2O when heated reduces to MgCO3.
weight of H2O = weight of MgCO3.nH2O - weight of MgCO3
= 15.67 - 7.58
= 8.09 g

Answer 3

When you subtract the 2 mass numbers, you have the mass of the water that was driven off.

Now to get the formula:

Convert the actual mass of MgCO3 (7.58g) to moles.

Convert the mass of the water to moles.

Make a ratio of the 2 moles and divide by the smaller number to get whole numbers.

Now you have the formula for the hydrate