Propanone can be made by heating calcium ethanoate, (CH₃COO)₂Ca, which gives propanone and calcium…

Step 1. Equation. $(\text{CH}_{3}\text{COO})_{2}\text{Ca} \longrightarrow \text{CH}_{3}\text{COCH}_{3} + \text{CaCO}_{3}$. Check the carbons: 4 on the left, $3 + 1$ on the right.

Step 2. $n(\text{propanone}) = \dfrac{5.80\ \text{g}}{58.08\ \text{g/mol}} = 0.0999\ \text{mol} = 99.9\ \text{mmol}$.

Step 3. The salt and the ketone are in a one to one ratio, so $m = 0.0999\ \text{mol} \times 158.17\ \text{g/mol} = 15.8\ \text{g}$.

Another way. Propan-2-ol with acidified dichromate, or propyne with water and mercury(II) sulphate, gives propanone from three carbons.