"When 0.10g of magnesium ribbon was dissolved in an excess of dilute HCl, the temperature rose by 4.3 degrees C.A separate experiment showed that the same volume of acid required 500J to raise the temperature by 1.0 degree C."

From this I was able to work out the enthalpy change per mole of magnesium. The second part of the question is the bit I'm stuck on...

"In a similar experiment, solid magnesium carbonate reacted with an excess of dilute hydrochloric acid and the enthalpy change was found to be -90kJ mol^-1 of magnesium carbonate.

The enthalpy change of the formation of H2O is -285kJ mol^-1 and of CO2 is -393kJ mol^-1. Use this data and your result from above to calculate the enthalpy change of formation of MgCO3."

Can you please talk me through how to do it?

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