Question

Question #d35f2

Answer 1

The student should use 1.0 × 10³ µL of clarified yeast extract.

The first question is, How much 100 % ethanol is needed to make 1.3 mL of 26 % ethanol?

This is a standard dilution calculation.

`"Volume" = "1.3 mL" × "26 %"/"100 %" = "0.34 mL"`

So you need to add enough clarified extract to make a total of 1.3 mL of solution.

How much extract is needed?

If there is no change of volume on mixing,

`V_"extract" = V_"total" – V_"ethanol" = "1.3 mL" – "0.34 mL" = "1.0 mL" = 1.0 × 10^3"µL"`

Note: The answer can have only 2 significant figures, because that is all you gave for the volume of the sample. If you need more precision, you will have to recalculate.

Answer 2

You would need `962` `muL` of clarified yeast extract.

Here's how you'd go about solving this problem. You know that you must mix ethanol and clarified yeast extract together to get a `"1.3-mL"` sample that is `"26%"` ethanol.

Now, I'm assuming you're dealing with a `"26% v/v"` solution, which is defined as

`"% volume" = V_("solute")/V_("solution") * 100`

If ethanol is your solute, a `"26% v/v"` solution would imply that every `"100 mL"` of solution contain `"26 mL"` of ethanol. As a result, your sample will contain

`"1.3 mL solution" * ("26 mL ethanol")/("100 mL solution") = "0.338 mL ethanol"`

This means that the volume of clarified yeast extract must be

`V_("solution") = V_("ethanol") + V_("yeast") => V_("yeast") = V_("solution") - V_("ethanol")`

`V_("yeast") = "1.3 mL" - "0.338 mL" = "0.962 mL"`, or

`"0.962 mL" * (1000 muL)/("1 mL") = "962"` `muL`

Rounded to two sig figs, the answer should be

`V_("yeast") = "960` `muL`