# Chemistry. What equation do I use?

A 20.29 g sample of impure MgCO3 is completely decomposed at 1000oC in previously evacuated 2.0 L reaction vessel. After the reaction was complete the solid residue had a mass of 15.9 g. Assume that only MgCO3 could produce gas CO2 what percent of original sample was magnesium carbonate? What was the pressure of the CO2 produced?

Do I use PV/T = PV/T to find this answer? Thank you for helping!

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1. You need to start from the chemical equation for the decompostion of MgCO3.

Assuming that all the CO2 resulted from the conposition of MgCO3, the mass difference tells you the mass of CO2 produced and hence the number of moles of CO2. The number of moles of CO2 can be used to
a) calculate the number of moles of MgCO3 decomposed (using the chemical equation). From the number of moles of MgCO3 calculate the mass of MgCO3 decomposed from which you can calculate the percentage of MgCO3 in the original sample.
b) I assume that the question is asking for the pressure at 1000oC. You can use
PV/T = PV/T
by assuming that 1 mole of any gas occupies 22.4 litres at STP. This give you the LHS values. For the RHS, V=2.0 L and T=1000oC (convert to kelvin), P we are trying to find.

Hope that this helps.

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posted by Dr Russ

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