Anybody know how to do this? I need help with it pleassseee?!?

Question

A 7 gram sample of a hydrated salt is heated over a bunsen burner until its mass drops to a constant 4.58 grams.

1. What is the percent by mass of water in the original sample.

2. If the formula for the anhydrous salt is ZnCl2, what is the formula for the hydrated salt?

Answer 1

1.) 7.00 grams "wet" mass - 4.58 grams "dry" mass =
2.42 grams of water.
2.42 grams of water in 7.00 grams originally =
34.57% by mass (2.42 / 7.00).

2.) 2.42 grams of water = 0.133 moles,
4.58 grams ZnCl2 = 0.033 moles
0.133 / 0.033 = 4.0
Four moles of water per mole of ZnCl2
ZnCl2 dot 4H2O.

Answer 2

[7.00 grams - 4.58 grams] ÷ [7.00 grams] = 34.57%

The general formula for zinc chloride hydrate is:

ZnCl2 ∙ n(H2O)

where the "n" refers to the number of moles of water per mole of zinc chloride. As it turns out, hydrated zinc chloride can exist in a few variations:

ZnCl2 ∙ 6(H2O), or;
ZnCl2 ∙ 7(H2O), for example.

In your case, we need to figure out how many moles of zinc chloride and how many moles of water you had to begin with, so that we can determine the final chemical formula for the zinc chloride hydrate.

We know that the sample originally contained 4.58 grams of ZnCl2 and 2.42 grams of H2O. To determine the number of moles of each, we simply need to know the molar mass of the two chemicals:

1) For ZnCl2, the molar mass is 136.27 grams/mol. So:
[4.58 grams] ÷ [136.27 grams/mol] = .034 moles ZnCl2

2) For water, the molar mass is 18.02 grams/mol. So:
[2.42 grams] ÷ [18.02 grams/mol] = .134 moles H2O

3) The ratio of ZnCl2 to H2O, then, is:
[.034 moles ZnCl2] ÷ [.134 moles H2O] = .253, or: roughly 1 mole of ZnCl2 for every mole of H2O.

4) Thus, the chemical formula for the original zinc chloride hydrate is: ZnCl2 ∙ 4(H2O).

Cheers!

Answer 3

Atomic Weights: Zn =65 Cl = 35.5

7.00 - 4.58 = 2.42gH2O
2.42/7.00 = 0.346 x 100% = 34.6% water

4.58gZnCl2 x 1molZnCl2/136gZnCl2 = 3.36*10^-2 molZnCl2
2.42gH2O x 1molH2O/18gH2O = 1.34*10^-1 molH2O = 13.4*10^-2 mol H2O
13.4*10^-2/3.36*10^-2 = 4
ZnCl2.4H2O

Answer 4

((7 - 4.58) / 7) * 100 = 34.57 %

The formula before and the formula after are the same.