SAT Subject Test Chemistry
REVIEW OF MAJOR TOPICS
Stoichiometry (Chemical Calculations) and the Mole Concept
A typical mass-volume problem:
EXAMPLE 1: How many liters of oxygen (STP) can you prepare from the decomposition of 42.6 grams of sodium chlorate?
This problem can also be solved using methods other than the proportion method shown above.
Another method to proceed from step 3 is called dimensional analysis, also referred to as the factor-label method. The reasoning is this: Because the equation shows that 213 grams of reactant produces 67.2 liters of the required product, multiplying the given amount by this equality (so that the units of the answer will be correct) will give the same answer as above. Then step 4 would be:
Still another method of solving this problem is called the mole method. Steps 1 and 2 are the same. Then step 3 is as follows:
EXAMPLE 2: Find the weight of CaCO3 needed to produce 11.2 L CO2 when the calcium carbonate is reacted with hydrochloric acid.
TRY THESE MASS-VOLUME PRACTICE PROBLEMS:
1. What mass of water must be electrolyzed to obtain 20.0 liters of oxygen at STP?
Ans. 32.2 g*
2. How many grams of aluminum will be completely oxidized by 44.8 liters of oxygen at STP?
Ans. 72.0 g*
* Answers explained at the end of this chapter.