A level Chemistry question on finding the mr of an unknown metal

I have been at this for a while and can’t do it. If someone could explain their method to getting the answer which is 24, I will be very grateful.

The metal is in the +2 oxidation state, since the salt contains the metal cation and CO3^2- anion in a 1:1 ratio and there are no other anions.

It tells you that the metal oxide forms on heating, and because the metal is in the +2 oxidation state, the formula of the metal oxide is MO.

Using the formula of the salt, MCO3.4H2O and the formula of the oxide, we get the following equation:

MCO3.4H2O —> MO + CO2 + 4H2O

15.6 grams of MCO3.4H2O was used and 4 grams of MO are formed, so the combined mass of CO2 and H2O formed is 11.6 g

Let’s treat the CO2 and H2O as one product and so assume that the reaction produces this one product in a 1:1 mole ratio with MO.

The combined Mr of CO2 and 4H2O = 44 + 4(18) = 116 g mol^-1

Therefore, (11.6 g)/(116 g mol^-1) = 0.1 mol, and as there is a 1:1 mole ratio of MO to this, there are 0.1 moles of MO.

Now that I’ve done the tricky part of the question, can you now:

(a) Use the mass of MO made and the moles of MO as found above to work out the relative formula mass of MO?

(b) Use the periodic table to look up the relative atomic mass of oxygen.

(c) Calculate the relative mass of M using your answers to (a) and (b).

Thank you for your reply! This is what I thought to do, but you say the ratio of MO: CO2 4H2O is 1;1 but wouldn’t it be 1:5?

You treat CO2 + 4H2O it like it is just a single compound (H8CO6, if you like), so it is 1:1.