Answer to Question #137417 in General Chemistry for Pamela

Question

You found an unknown hydrate in the laboratory. From the labels you found on the floor, you guessed this unknown must be either cobalt choride (CoCl₂·2H₂O) or oxalic acid (H₂C₂O₄·2H₂O). You performed an experiment similar to what you did in this laboratory. When a 0.750-g sample of the unknown hydrate was heated, the resulting anhydrous compound had a mass of 0.536 g. Determine what unknown you found in the laboratory and explain your answer by showing your work to support your explanation

Answer 1

Molar mass of CoCl₂.2H₂O = 165.87g/mol

Molar mass of H₂C₂O₄.2H₂O = 126 g/mol

moles of $CoCl_2.2H_2O$ = $0.75\div 165.87 =0.00452$ moles

moles of $H_2C_2O_4.2H_2O = 0.75 \div 126 = 0.00595$ moles

mass of H₂O = mass of hydrated - mass of dehydrated compound

mass of H₂O = 0.75 - 0.536g = 0.214 g

Moles of H₂O = $0.214 \div 18 = 0.0119$ moles

Let's say

X.2H₂O $\rightarrow X + 2H_2O$

It means for 1 mole of hydrated compound it gives 2 moles of water

since upon dehydration of 0.00595 moles of oxalic acid it gives 0.0119 moles which is correct.

So the unknown compound is $H_2C_2O_4.2H_2O.$