Chemistry Help Questions

Question

Please help in showing how you solve.

1. Calculate the percent composition, by mass, of calcium phosphite. Show the formula.

2. A compound is 74.1% carbon, 8.6% hydrogen, and 17.3% nitrogen by mass. It has a molecular mass of 160

g/mol. What is the molecular formula for this compound?

3. A calcium chloride hydrate has a mass of 4.72 g. After heating for several minutes the mass of the anhydrate is found to be 3.56 g. Use this information to determine the coefficient for the hydrate and name the hydrate.

Answer 1

For (1), the problem asks for percent composition of calcium phosphite. The first thing to check is the chemical formula.

Calcium phosphite = Ca₃P₂

To get the percent composition, we are trying to answer the question of how much % of the total mass belongs to calcium and how much belongs to phosphorus. % composition = g of element/ g of compound.

Let's list out all the molar masses necessary:

Ca3P2 = 182.182 g/mol, Ca = 40.078 g/mol, P = 30.974 g/mol

% composition of Ca = 3 mol (40.078) / 182.182 = 66%

% composition of P = 2 mol (30.974) / 182.182 = 34%

For the next problem (2), we are approaching the concept from the opposite direction. Here, the key assumption is to assume you have 100 g of this mystery compound. This means you have 74.1 g of carbon, 8.6 g of hydrogen, 17.3 g of nitrogen. We need to convert these grams to moles, since that's the unit that chemical formulas are in.

74.1 g C / 12.011 g/mol = 6.169 mol C

8.6 g H / 1.008 g/mol = 8.532 mol H

17.3 g N / 14.007 g/mol = 1.235 mol N

So far, the chemical formula is C_6.169H_8.532N_1.235 which is not how we report formulas. To make it look nice, divide everything by the smallest number.

C₅H₇N

Now, we can use the last piece of information the problem gave us, that the molar mass of this compound is 160 g/mol. Does the emprical formula above match 160 g/mol? No, the molar mass of the empirical formula is 81 g/mol. This means we have to double all the subscripts.

C₁₀H₁₄N₂ = 162 g/mol.

For problem (3), we are told how much water has been evaporated off the salt by heating (hydrate --> anhydrate). We went from 4.72 g to 3.56 g. This means we lost 1.16 g of water! To relate this to the chemical formula, convert to moles.

1.16 g water / 18 g/mol = 0.0644 mol water lost

We need to relate this mol number to the moles of CaCl₂ x H₂O. Let's list the molar mass of the anhydrous version, CaCl₂. 110.978 g/mol. Now we can figure out how many moles of CaCl₂ was left behind.

3.56 g / 110.978 g/mol = 0.0321 mol CaCl₂.

To figure out the ratio between the salt and the water, we compare the two mole values we just calculated.

0.0321 vs 0.0644

1 vs 2

Therefore, the formula of the hydrated salt is CaCl₂ 2H₂O, or calcium chloride dihydrate.