Grant found the mass of an empty crucible and lid to be 62.549 g. After he added an unknown, the mass was 63.089 g. The crucible and lid and unknown were
heated strongly for 10 minutes. After cooling, the mass of the crucible and lid and sample
were found to be 62.928 g. Grant also performed a flame test on a sample of his unknown,
and found it produced a violet flame. Is the unknown NaHCO3, Na2CO3, KHCO3, or K2CO3?
Explain the reasoning used to arrive at your answer.

See explanation

Explanation:

First of all, we have been told that Grant performed a flame test on the  unknown sample and the flame colour was found to be violet. Violet flame implies that the compound is a compound of potassium metal since potassium metal produces a violet flame.

We can see that the mass of crucible + sample decreased from 63.089 g to 62.928 g implying that the unknown sample was decomposed and some part of it may have been lost as gases.

Having this in mind, we already know that KHCO3 decomposes according to the reaction; 2 KHCO3(s) → K2CO3(s) + CO2(g) + H2O(g)

So, the unknown compound must be KHCO3. Decomposition of the compound involves the loss of H2O and CO2 as gases which accounts for the decrease in mass of crucible + unknown sample after heating.