Working with impure chemicals - PowerPoint Presentation

Slide1

Working with impure chemicals

How do I perform calculations when my reagents are not completely, 100% pure?Slide2

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:

10.000 g

Here, we show 10.000 grams of ‘pure’ NaCl. This means that ALL of the powder in our 100% pure NaCl is, in fact, NaCl.Slide3

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:

10.000 g

Here, we show 10.000 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”.Slide4

Why does using an “impure”

chemical affect our calculations?Suppose that we know that we need 10.000 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:

10.000 g

Here, we have weighed out 10.000 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:

9.000 g

1.000 g

For every 10.000 grams of 90% NaCl, we only have 9.000 grams of NaCl.

Therefore, we must weigh out MORE than 10.000 g of 90% NaCl to get the 10.000 grams of NaCl we need!Slide5

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!!Slide6

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….Slide7

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

? =

21.05

g

95% pure

NaCl

is needed