# Question Video: Calculating the Percentage Mass of a Hydrated Salt in a Mixture Chemistry

A chemist recovers a 7.56 g mixture containing solid CuCl₂⋅2H₂O and NaCl. The chemist heats up the mixture to remove the water molecules until a constant mass of 6.58 g is obtained. What is the percentage mass of CuCl₂⋅2H₂O in the mixture? Give your answer as an integer. [Cu = 63.5 g/mol, Cl = 35.5 g/mol, H = 1 g/mol, O = 16 g/mol, Na = 23 g/mol]

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### Video Transcript

A chemist recovers a 7.56-gram mixture containing solid CuCl2⋅2H2O and NaCl. The chemist heats up the mixture to remove the water molecules until a constant mass of 6.58 grams is obtained. What is the percentage mass of CuCl2⋅2H2O in the mixture? Give your answer as an integer. The molar mass of copper is 63.5 grams per mole. Chlorine is 35.5 grams per mole. Hydrogen is one gram per mole. Oxygen is 16 grams per mole. And sodium is 23 grams per mole.

In this question, we want to know what the mass percentage is of CuCl2⋅2H2O, or copper(II) chloride dihydrate, in this mixture. The mixture containing copper(II) chloride dihydrate and sodium chloride has a total mass of 7.56 grams. Copper(II) chloride dihydrate is an example of a hydrated salt, which is a salt that contains molecules of water within its crystal structure. NaCl does not contain any molecules of water within its crystal structure and so is known as an anhydrous salt.

Upon heating a hydrated salt, molecules of water are lost from the crystal structure. As a result, the mass of the salt decreases, and the anhydrous salt is formed. We are told that after being heated, the mass of the mixture decreases to 6.58 grams. This decrease in mass results from the loss of water. Therefore, if we subtract the mass after heating from the mass before heating, we will get the mass of water lost, which is 0.98 grams.

We can use this information to help us determine the mass of CuCl2⋅2H2O in the mixture. Let’s start by calculating the number of moles of water molecules present in the mixture. To do so, we will need to divide the mass of water by the molar mass of water. Dividing 0.98 grams by the molar mass of water, which is 18 grams per mole, gives us the amount of moles of water.

Now, to find the number of moles of CuCl2, let’s take a closer look at the chemical formula of the hydrated salt. We can see that for every two moles of water, there is one mole of copper(II) chloride. Therefore, we can calculate the number of moles of copper(II) chloride by dividing the number of moles of water by two. Let’s wait to round our answer until the last step of the problem. Now we can rearrange the same equation we used earlier to determine the mass of copper(II) chloride present in the mixture. Let’s multiply the number of moles of copper(II) chloride by its molar mass, which is 134.5 grams per mole.

Now that we have the mass of copper(II) chloride in the mixture, we can use it to determine the mass of the hydrated salt in the mixture. To do so, let’s add together the masses of copper(II) chloride and water. Finally, we’re ready to calculate the percentage mass of the hydrated salt in the mixture. We need to divide the mass of the hydrated salt by the total mass of the mixture and multiply by 100 percent. Next, we must round our answer to the nearest integer. This gives us a value of 61 percent.

In conclusion, the percentage mass of CuCl2⋅2H2O in the mixture is 61 percent.