[sinthreck]

You have a stock solution of glycine, 100mg/L. How many ml of glycine and how many ml of water are required to make a 25mL solution of 4mg/L Glycine

My solution:

100/4 = 25

Therefore you need 25 times the amount of water to dilute the glycine to 4mg/L

Let x be the number of mL you require of glycine.

25x + x = 25
x = 0.9615 mL

Answer:
Glycine: .9615 mL
Water: 24.04mL

Is this answer correct?

[sjb]

You have a stock solution of glycine, 100mg/L. How many ml of glycine and how many ml of water are required to make a 25mL solution of 4mg/L Glycine

My solution:

100/4 = 25

Therefore you need 25 times the amount of water to dilute the glycine to 4mg/L

Let x be the number of mL you require of glycine.

25x + x = 25
x = 0.9615 mL

Answer:
Glycine: .9615 mL
Water: 24.04mL

Is this answer correct?

Glycine is a solid at room temperature and pressure, so you may need a density to convert between volume and mass. However, it is not clear what you mean by "a 25mL solution of glycine". Does this mean your total volume is 25 ml, or that the volume of glycine within your sample is 25 ml? Can you clarify here?

(edit; as I'm sure it didn't say 25ml solution of 4mg/L Glycine before  )

If the initial concentration is 100 mg/L, then you need to dilute the original concentration 1 part in 25 to bring it down to the required. So you would take 1 ml of stock and add enough water to make it 25 ml, not 25 times the amount of water compared to stock.

[Borek]

Glycine is a solid at room temperature and pressure, so you may need a density to convert between volume and mass.

It doesn't matter. Question asks to start with stock solution, 100mg/mL.

Glycine: .9615 mL
Water: 24.04mL

Lets check. You take 0.9615 mL of glycine solution, so it contains 0.9615mL*100mg/1000mL=0.09615mg, and it is dissolved in 25mL, so the concentration is 0.09615mg/0.025L=3.846mg/mL. Not good.

How many mg do you need in the final solution? In what volume of stock solution you will have this amount?

[sinthreck]

Borek,

Using your calculation, I worked out the solution in a backwards manner.

1mL solution will contain 1mL*100mg/1000mL = .1mg and it is dissolved in 25mL, so the concentration is .1mg/.025L = 4mg/L which is exactly what I was looking for.

I'm still not satisfied, however, as this was working it out backwards and using a calculator.

I stuffed up this question in the exam, I spent around 20 minutes on it and couldn't figure it out. No calculators were allowed in the exam. It was so basic, I can't believe it tripped me up!

[Borek]

I asked how many mg of glycine are present in 25 mL of 4 mg/1L solution, I am no sure what you did and how it is related to what wrote.

[sinthreck]

ok,

so without using a calculator:

4mg /1000mL
1mg / 250mL
.1mg / 25mL

There are .1mg in 25mL of a glycine solution

[Borek]

So you need 0.1 mg - how many mL of 100mg/L solution will contain this amount? Start with that volume, fill up to 25 mL, you are done.

[sinthreck]

Yes thanks.

It's so easy, can't believe I couldn't get it in the exam.

We need .1mg

We have a 100mg/L stock solution.

Which is 100mg/1000ml
1mg/10ml
.1mg per 1ml

The solution is 1mL of glycine and 24mL of water.

Thanks Borek