Question

(b) In another experiment Penny obtained white crystals of potassium carbonate, K₂CO3,

from the wood ash.
(1)
Show that the percentage by mass of potassium in K₂CO3 is 56.6.
[2]
(ii) Some of Penny's crystals were analysed for potassium by flame emission
spectroscopy. The results showed that the percentage of potassium present was
44.9%.
Penny suggested that the crystals of potassium carbonate might be a hydrate,
K₂CO3.2H₂O.
Explain why the percentage of potassium in the hydrate is lower than the value
stated in (i).
[1]

Answer 1

Step-by-step explanation:

(1) To show that the percentage by mass of potassium in K2CO3 is 56.6%, we can use the formula weight of the compound. The formula weight of K2CO3 is 138.20 g/mol. The molar mass of potassium is 39.10 g/mol and the molar mass of carbon and oxygen is 99.10 g/mol. Therefore, the percentage by mass of potassium in K2CO3 is (2 x 39.10) / 138.20 x 100 = 56.6%.

(ii) The percentage of potassium in the hydrate is lower than the value stated in (i) because the hydrate contains water molecules, which also contribute to the overall mass of the compound. Therefore, the percentage of potassium in the hydrate is diluted by the mass of the water molecules and will be lower than the value for the anhydrous compound.