Working with impure chemicals How do I perform calculations when my reagents are not completely, 100% pure?

What do we mean by a “pure” chemical? Sometimes when we work with chemicals, our chemicals are ‘pure’, which means that ALL of the liquid or solid is the chemical of interest: g Here, we show grams of ‘pure’ NaCl. This means that ALL of the powder in our 100% pure NaCl is, in fact, NaCl.

What do we mean by an “impure” chemical? An impure chemical contains the desired chemical, as well as some amount of other substances. Therefore, in the case of ‘impure’ chemicals NOT ALL of the liquid or solid is the chemical of interest: g Here, we show grams of ‘90% pure’ NaCl. This means that 90% of the powder in our 90% pure NaCl is, in fact, NaCl. The other 10%, shown here as little black dots, is what we call “inert filler”.

Why does using an “impure” chemical affect our calculations? Suppose that we know that we need grams of NaCl to make a solution. Obviously, the easiest thing to do would be to use 100% pure NaCl. But we only have 90% pure NaCl. Let’s see what this means: g Here, we have weighed out grams of ‘90% pure’ NaCl. Imagine that we could actually separate out the “filler” from the NaCl (which we can’t). Let’s see what this would look like: g g For every grams of 90% NaCl, we only have grams of NaCl. Therefore, we must weigh out MORE than g of 90% NaCl to get the grams of NaCl we need!

So how do we decide how much of an ‘impure’ chemical to add? Use this ratio: % purity = units of desired chemical 100 units of impure chemical For example: 90% pure NaCl = 90 g NaCl 100 g impure NaCl Note that your “units” can be anything (g, mg, mL etc) as long as they are the same on the top and bottom of your ratio!!

Let’s do a problem: You need to make up 2 L of 10 g/L NaCl. You only have 95% pure NaCl. How much 95% pure NaCl must you measure out? First, how much NaCl (imagine it is pure) do you need? 10g= ? 1 L 2 L ? = (2 L)(10 g) 1 L ? = 20 g NaCl is needed Continue….

Now, to be sure you add 20 g of NaCl, how much 95% pure NaCl do you need? % purity = units of desired chemical 100 units of impure chemical Therefore, 95% pure NaCl = 95 g NaCl 100 g of 95% pure NaCl You need 20 g NaCl, so: 20g NaCl= 95 g NaCl ? 100 g of 95% pure NaCl (20 g NaCl)(100 g of 95% pure NaCl) = ? 95 g NaCl ? = g 95% pure NaCl is needed