CHM 2045C Module Five Sample     Name:_______________

Module Four Part N Mixture Problem  4 points

Problem #77 p 172 Kotz 6th Edition (5th Edition Problem 65 with different numbers)

Sodium hydrogen carbonate, NaHCO3  can be decomposed quantitatively by heating:

2 NaHCO3 (s) à Na2CO3 (s) +  CO2 (g) +  H2O (g)

A 0.682 g sample of impure NaHCO3 yielded a solid residue (consisting of Na2CO3 and other solids) with a mass of 0.467 g.  What was the mass percent of NaHCO3 in the sample?

1. Let x = mass of pure NaHCO3 in the original sample

2. Let y = mass of impurities in the original sample

3. Therefore: x + y = 0.682 g mixture or y = 0.682 - x

4. Calculate how much Na2CO3 can be produced from x grams of NaHCO3

5. Assuming the impurities before the heating remain as solid impurities after heating,

0.631x g Na2CO3  +  y g impurities  =  0.467 g after heating mixture or

0.631x + y = 0.467

6. From #3  y = 0.682- x and substituting in #5 yields the equation:

0.631x + (0.682 – x) = 0.467

0.631x – x = 0.467 – 0.682

- 0.369x  = - 0.215

X = - 0.215 / - 0.369  =  0.583 g pure NaHCO3

0.583 g pure/0.682g impure x 100 =  85.4 % pure

Additional Mixture Problems:

Q# 75 p 171 Kotz 6th Edition

Q#78 p 172 Kotz 6th Edition