**Problem #3:** For the reaction represented by the equation:

CX_{4} + 2 O_{2} ---> CO_{2} + 2 X_{2}O

9.0 g of CX_{4} completely reacts with 1.74 g of oxygen. What is the approximate molar mass of X?

**Solution #1**

1) moles O_{2} = 1.74 g ÷ 32.0 g/mol = 0.0544 mol.

2) The CX_{4} to O_{2} ratio is 1 to 2. Therefore, 1 is to 2 as x is to 0.0544. x = 0.0272 mol of CX_{4} reacted.

3) 9.0 g ÷ 0.0272 mol = 331 g/mol. This is the molecular weight of CX_{4}

4) Subtract 12 from 331 to get 319, which is the weight contribution of X_{4}.

5) Dividing by four gets 79.8. The nearest atomic weight on the periodic table is that of bromine. The formula is CBr_{4}.

Here are some some pieces of alternate solutions that use bits of the above solution and then go off in a "different" direction. In each case, solve for x.

(A) 9.0 g ÷ (12.011 + 4x) g/mol = 0.0272 mol

(B) (1.74 g / 32.0 g/mol) ÷ (9.0 g / [12 + 4x] g/mol) = 2 ÷ 1

**Solution #2**

I would have NEVER thought of this:

1) 64.0 /1.74 = 36.8 (I think the proper unit here is mol¯^{1})

2) 36.8 mol¯^{1} x 9.0 g = 331 g/mol

Pretty wild, huh? The two steps could be put together this way: 64.0 ÷ 1.74 = x ÷ 9. By the way, the 64.0 comes from the 2O_{2}.

**Solution #3**

1) 1.74 g is the 2O_{2} in the equation. Half of it goes to the CO_{2}. This is 0.87 g of oxygen.

2) In CO_{2}, there is 72.7% oxygen (32 ÷ 44), so (0.727) (x) = 0.87. x = 1.196 g, the total mass of CO_{2} which contains 0.87 g of oxygen.

3) Carbon is 27.27% carbon (12 ÷ 44) in CO_{2}, so (0.2727) (1.196) = 0.326 g of carbon in CO_{2}.

4) 0.326 g of carbon is 0.02717 mol of carbon. This is also the mol of CX_{4} present.

5) 9.0 minus 0.326 equals 8.674g, the mass of X_{4} in CX_{4}. 8.674 ÷ 4 = 2.1685 g, the mass of one X in the 9.0 g sample.

6) 2.1685 g ÷ 0.02717 mole = 79.8 g/mol, the atomic weight of bromine.

**Problem #4:** A mixture of NaCl and NaBr weighing 1.234 g is heated with chlorine gas, which converts the mixture completely to NaCl. The total mass of NaCl is now 1.129 g. What are the mass percentages of NaCl and NaBr in the original sample?

**Solution**

1) The difference in the sample weights is due to the difference in atomic weights between Br and Cl:

79.904 − 35.453 = 44.451 g/molOne mole of the

differenceweighs 44.451 g

2) The difference in mass after heating is complete:

1.234 − 1.129 = 0.105 g

3) The mass difference of 0.105 g is due to difference in atomic weights between Br and Cl.

4) Calculate moles of NaBr converted to NaCl:

0.105 g / 44.451 g/mol = 0.00236215 mol

5) Calculate grams of NaBr:

(0.00236215 mol) (102.894 g/mol) = 0.243 g

6) Determine mass percent of NaBr in original sample:

0.243 g / 1.234 g = 19.7% (to three sig figs)mass percent of NaCl would be 80.3%