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# any chemistry students help with GLC question..

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1. I am stuck on this question its about using GLC to determine ethanol concentration in the blood.

If you had a blood sample of 5.71 mL, how many uL of n-propanol would you need to add
to achieve a concentration of 100 mg/100 mL ? The density of n-propanol is 0.803 g/mL

I am not sure if i need to use the volume of blood in this or not. I thought not so just worked out how much of the n-propanol i would need to get the concentration.

I used c1xv1=c2xv2
so converted all the units to the same so..

803xv1= 100x100
therefore v1 = 12.45ml = 12450uL.

is this right?
2. The concentration needed is 1000mg/1000ml

The volume of 1000mg of n-propanol is 1/0.803 = 1.25ml

So you need to add 1.25ml or 1250ul of n-propanol to 1000ml of blood to get the require concentration.

The volume of n-propanol needed to make 5.7ml have a concentration of 1000mg/1000ml is 1250*5.7/1000=7.09ul

Your answer of 12450ul is 12.450ml which is larger than the volume of the blood. You need to see if your answers makes sense and this clearly dosen't.
3. (Original post by Maker)
The concentration needed is 1000mg/1000ml

The volume of 1000mg of n-propanol is 1/0.803 = 1.25ml

So you need to add 1.25ml or 1250ul of n-propanol to 1000ml of blood to get the require concentration.

The volume of n-propanol needed to make 5.7ml have a concentration of 1000mg/1000ml is 1250*5.7/1000=7.09ul

Your answer of 12450ul is 12.450ml which is larger than the volume of the blood. You need to see if your answers makes sense and this clearly dosen't.
the conc. needed is 100mg/100ml would this make any difference?
4. (Original post by Smile-Like-You-Mean-It)
the conc. needed is 100mg/100ml would this make any difference?
No, 100mg/100ml is the same concentration as 1000mg/1000ml
5. I confused! Could you explain this?!
6. (Original post by StrawberryTart)
I confused! Could you explain this?!
What do you what explaining?
7. (Original post by Maker)
What do you what explaining?
Why is this equation used? Why divide by 1000?
1250*5.7/1000=7.09ul

Thanks
8. I just prefer working in 1000ml, the answer is the same. Actually, it would be simpler to work in mL.

So if you wanted to get a concentration of 1ul/mL of n-propanol which is the same concentration as 100ul/100mL, you would add;

1/0.803 x 5.7 = 7.09ul. n-propanol

You will have noticed the answer is slightly wrong because you have increased the volume of the sample slightly but the answer is accurate enough in this case. If the volume of n-propanol was larger in proprtion to the volume of the sample, you would need to take it into account to prevent significant errors.
9. (Original post by Maker)
No, 100mg/100ml is the same concentration as 1000mg/1000ml
hey, i followed your steps but the first step 1/0.803 for 1000mg/1000ml, wouldnt it change if it were 100mg/100ml as isnt the 1, 1g as 1000mg = 1g?

I did,
0.1g/0.803g/ml = 0.125ml
0.125mlx5.7ml = 0.7125ml = 712.5ul?
10. (Original post by Smile-Like-You-Mean-It)
hey, i followed your steps but the first step 1/0.803 for 1000mg/1000ml, wouldnt it change if it were 100mg/100ml as isnt the 1, 1g as 1000mg = 1g?

I did,
0.1g/0.803g/ml = 0.125ml
0.125mlx5.7ml = 0.7125ml = 712.5ul?
Lets see if I can explain it better.

We want a sample with a concentration of n-propanol of 100mg/100ml.

This would be equal to 1mg/ml because the concentrations are the same. Converting it to 1mg/ml makes the calculations a bit easier.

A concentration of 1mg/ml is the same concentration as 100mg/100ml which is the same as 1000mg/1000ml.

We are told that density of n-propanol is 0.803g/ml. We are asked for how much volume of n-propanol has to be added. To convert density to volume, Use the formula d=m/V

0.803g/ml= 1/V , V=1/0.802 =1.25ml

1.25ml of n-propanol weights 1g.

To get to a concentration of 100mg/100ml or 1mg/ml in 5.7ml of blood, we need 5.7mg of n-propanol.

1mg of n-propanol has a volume of 1.25ml/1000= 0.00125ml or 1.25ul

The volume of 5.7mg of n-propanol is

5.7x1.25ul = 7.125ul.

So you need to add 7.125ul of n-propanol to 5.7ml of blood to get a concentration of 100mg/100ml

I noticed I've given a wrong answer before. No wonder you were confused. My apologies
11. (Original post by Maker)
Lets see if I can explain it better.

We want a sample with a concentration of n-propanol of 100mg/100ml.

This would be equal to 1mg/ml because the concentrations are the same. Converting it to 1mg/ml makes the calculations a bit easier.

A concentration of 1mg/ml is the same concentration as 100mg/100ml which is the same as 1000mg/1000ml.

We are told that density of n-propanol is 0.803g/ml. We are asked for how much volume of n-propanol has to be added. To convert density to volume, Use the formula d=m/V

0.803g/ml= 1/V , V=1/0.802 =1.25ml

1.25ml of n-propanol weights 1g.

To get to a concentration of 100mg/100ml or 1mg/ml in 5.7ml of blood, we need 5.7mg of n-propanol.

1mg of n-propanol has a volume of 1.25ml/1000= 0.00125ml or 1.25ul

The volume of 5.7mg of n-propanol is

5.7x1.25ul = 7.125ul.

So you need to add 7.125ul of n-propanol to 5.7ml of blood to get a concentration of 100mg/100ml

I noticed I've given a wrong answer before. No wonder you were confused. My apologies
oh thanks, i think i get it now!!
thankyou xxx

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