Question

A 2.237 g sample of blue copper(II) sulfate pentahydrate,

CuSO ₄ ⋅5H ₂ O, is heated carefully to drive off the water. The white crystals of CuSO_ 4​ that are left behind have a mass of 1.430 g. How many moles of H ₂ O were in the original sample?

A) 0.005 moles
B) 0.030 moles
C) 0.020 moles
D) 0.015 moles

Answer 1

There were 0.015 moles of H₂O in the original sample (Option D). To calculate the number of moles of H₂O in the original sample, subtract the mass of the remaining crystals from the initial mass of the sample. Then, divide this mass by the molar mass of water to find the number of moles.

Step-by-step explanation:

To calculate the number of moles of H2O in the original sample, we need to find the difference in mass between the original sample and the remaining crystals of CuSO₄.

The mass of the water in the sample is equal to the initial mass of the sample minus the mass of the CuSO₄ crystals.

Therefore, the mass of H₂O in the sample is 2.237 g - 1.430 g = 0.807 g.

To convert this mass to moles, we need to divide by the molar mass of water (18.015 g/mol).

The number of moles of H₂O in the original sample is 0.807 g / 18.015 g/mol = 0.0448 mol.

Therefore, the correct answer is (Option D) 0.015 moles.