Working out the dilution
When you're thinking about dilution, it helps to simplify your actions into dilution factors. When we said the diluted coffee was
"1/10th as strong as the original"
we were explaining how much the coffee was diluted. We could also have said
“the coffee was diluted 1/10”, or
“the coffee was diluted 10-1”, or, less frequently,
“the coffee was diluted 0.1”
Here are some more examples for you to try:
- 1 mL coffee + 4 mL water =
- 1 mL coffee + 9mL water =
- 1 mL + 99mL water =
As you've probably guessed, this works exactly the same whether you're talking about caffeine or bacterial cells. Here's what a dilution of 1/100, or 10-2, looks like on the lab bench:
Notice that it really doesn't matter how much of the original stock you started with, as long as you had enough to put 1 mL into the new container. What matters is how much you transfer and how much water you add. The dilution is then defined as:
Volume transferred / total volume
where the total volume is the volume transferred + volume of water added. Therefore,
1 / (1+99) = 1/100 = 10-2 = 0.01.
As shown above, the dilution can be expressed as a fraction (1/100), in scientific notation (10-2) or, less commonly, as a decimal (0.01). In this example, water is the diluent (a general term for the liquid used to dilute the sample).
In the applet below, give the dilution factor as a decimal:
What is the dilution factor?
1 mL stock + mL water
Can you do it in your sleep? OK, then go on to the next page...
photo credits: sleeping student |
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