E29 Fundamentals

[MichelleTran3I]

A chemist measured out 8.61 g of copper(II) chloride tetrahydrate, CuCl2 4H2O. (a) How many moles of CuCl2 4H2O were measured out? (b) How many moles of Cl ions are present in the sample? (c) How many H2O molecules are present in the sample? (d) What fraction of the total mass of the sample was due to oxygen?

for part b, how would you determine how many moles of Cl ions are present? I am also confused on how to approach part d.

[Riya Pathare 2E]

For part be you would take the number of moles you have for copper chloride tetra hydrate and multiply it by its proportionality factor to chlorine. Since every mole of copper chloride tetra hydrate has 2 moles chlorine you would multiply it by (2 mol Cl/1 mol CuCl2 4H2O). For part d calculate the mass of the oxygen in the compound and divide it by the compound's total mass to find what percent it is.

[Maeve Gallagher 1J]

For part b, you look at molar ratios to determine the moles of Cl ions present. From the molecular formula you can see that for every 1 mole of CuCl2 x 4H2O, there are 2 moles of Cl ions. So you just double the number you found in part a and that will give you the moles of Cl. For part d, you find the total moles of oxygen again from the molecular formula, which would be 4 (from 4H2O). You then multiply 4 by the molar mass of oxygen (16g/mol), and divide the product by the total mass of the copper chloride tetrahydrate. This will give you the fraction of oxygen mass out of the total mass. Hope thats helpful!

[Vincent Grospe 3C]

For Part B, you must use formula stoichiometry. For every mole of CuCl2 4 H20, there are two moles of Cl- ions (as well as 1 mole of Cu2+ and 4 moles of H20) based on the formula of the compound. Therefore, after finding the the number of moles of the compound, you will multiply it by 2 since the ratio of moles of Cl- ions to that of CuCl2 4 H20 is 2:1.

In regards to Part D, you are fractionally finding how much mass does oxygen occupy in the entire compound. So you divide the molar mass of the amount of oxygen in the compound (4 x the molar mass of oxygen -- 4 is from the formula) by the molar mass of the entire compound.

Hope this helps!