Chapter 20 Laboratory: Quantitative Analysis 355

Laboratory:

Quantitative Analysis

In the last chapter, we looked at qualitative analysis, which is used to identify the chemical

species present in an unknown. After you identify which species are present, the obvious

next step is to determine the relative amounts of each species that are present. The methods

used to make that determination are collectively called quantitative analysis.

The key difference between qualitative analysis and quantitative analysis is that the

former is not concerned with quantities. If you do a qualitative analysis on an unknown for

the presence of potassium, for example, you care only whether potassium is present or

absent. If you do a quantitative analysis on that sample, your goal is to determine how much

potassium is present.

If you’ve completed all of the laboratory sessions in the preceding chapters, you’ve

already done one quantitative analysis without even being aware of it. In Chapter 11, we

standardized a solution of hydrochloric acid. In other words, we determined how much

hydrochloric acid was present in a sample. In doing that, we did a quantitative analysis.

Because the goal of quantitative analysis is to determine amounts, it’s critical to measure

masses and volumes as accurately as possible. In the preceding chapter on qualitative

analysis, we often gave instructions like “transfer about 5 mL of the sample,” because the

exact amount didn’t really matter. In this chapter, the instructions are considerably more

precise—such as “transfer 5.00 mL of the sample”—because for quantitative analysis, the

amount is critical. The lab sessions in this chapter reflect that careful attention to quantities.

20