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### Topic: Volume calculations  (Read 439 times)

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#### mrtinkles

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• Mole Snacks: +0/-0 ##### Volume calculations
« on: November 05, 2020, 05:25:17 AM »
Hello, is anyone able to help on how to go about doing this?

An experiment requires a solution which is 160 mM glycine and 10 mM MgSO4. What you have available in the lab is distilled water, a 2.2 M stock solution of glycine, and a 0.5 M MgSO4  stock solution. The experiment requires 5 ml of the solution in a test tube.

(a) What volume of the stock glycine solution do you need to transfer to the test tube?

(b) What volume of stock MgSO4  solution do you need to transfer to the test tube?

(c) What volume of distilled water do you need to transfer to the test tube?

(d) How many moles of glycine are in the test tube?

(e) How many mg of glycine does this represent?  (The mass of 1 mole of Glycine is 75g.)

#### chenbeier

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« Reply #1 on: November 05, 2020, 06:23:13 AM »
Where are the problems? Show your work First.

#### mrtinkles

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« Reply #2 on: November 11, 2020, 09:05:43 AM »
The problem is I dont know what I'm meant to be doing with it so unsure where to start. It would be nice if someone could explain how to do the first one so I can work from there

#### AWK

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« Reply #3 on: November 11, 2020, 09:51:28 AM »
The problem is with diluting the solutions. Every general chemistry textbook does this.
AWK

#### chenbeier

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« Reply #4 on: November 11, 2020, 09:56:36 AM »
The problem is I dont know what I'm meant to be doing with it so unsure where to start. It would be nice if someone could explain how to do the first one so I can work from there

Start first you have to do a 5 ml solution. Calculate the mole of Glycine and magnesiumsulfate with the given target values.
If you have this  how much ml you need by using the stocksolutions. The last is to calculate the water by subtraction of all volume from 5 ml.

#### mrtinkles

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« Reply #5 on: November 11, 2020, 10:33:24 AM »
How does this look?

An experiment requires a solution which is 160 mM glycine and 10 mM MgSO4. What you have available in the lab is distilled water, a 2.2 M stock solution of glycine, and a 0.5 M MgSO4  stock solution. The experiment requires 5 ml of the solution in a test tube.

(a) What volume of the stock glycine solution do you need to transfer to the test tube?

160mM / 1000 = 0.16M

C1V1 = C2V2
= 2.2M x V1 = 5ml x 0.16M      /   2.2M
= v1 = 5ml x 0.16M / 2.2M
= 0.36ml

V1 - V2
= 5ml - 0.36ml = 4.64ml water needed (without other components)
?

Thanks

#### AWK

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« Reply #6 on: November 11, 2020, 11:02:52 AM »
Correctly, you add water to the final volume. In the case of relatively dilute solutions, you may assume the additivity of volumes.
AWK

#### mrtinkles

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« Reply #7 on: November 11, 2020, 01:25:04 PM »
Okay thanks, so the calculations from a - c should hopefully look like this?

An experiment requires a solution which is 160 mM glycine and 10 mM MgSO4. What you have available in the lab is distilled water, a 2.2 M stock solution of glycine, and a 0.5 M MgSO4  stock solution. The experiment requires 5 ml of the solution in a test tube.

(a) What volume of the stock glycine solution do you need to transfer to the test tube?

160mM / 1000 = 0.16M glycine

C1V1 = C2V2
= 2.2M x V1 = 5ml x 0.16M      /   2.2M
= V1 = 5ml x 0.16M / 2.2M
= 0.36ml of glycine

(b) What volume of stock MgSO4  solution do you need to transfer to the test tube?

10mM / 1000 = 0.01M MgSO4

C1V1 = C2V2
= 0.5M x V1 = 5ml x 0.01M      /   0.5M
= V1 = 5ml x 0.01M / 0.5M
= 0.1ml of MgSO4

(c) What volume of distilled water do you need to transfer to the test tube?

V1 - V2
= 5ml - 0.46ml
= 4.54ml water needed

#### chenbeier

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« Reply #8 on: November 11, 2020, 02:10:05 PM »

#### mrtinkles

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« Reply #9 on: November 11, 2020, 02:29:28 PM »
I dont really understand it but if I needed 0.46 ml of the chemicals then would 4.54ml of water not be right to make it up to 5ml?

#### chenbeier

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« Reply #10 on: November 11, 2020, 02:33:19 PM »
During mixing volume contraction or expansion  can happen volume of solutions can not added. This is only valid for water.

#### mrtinkles

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« Reply #11 on: November 11, 2020, 02:58:27 PM »
Oh thanks I get it now.

I'm trying to work out the next two questions now.

(d) How many moles of glycine are in the test tube?
I suppose this is using the information I just worked out but not sure in which way?

(e) How many mg of glycine does this represent?  (The mass of 1 mole of Glycine is 75g.)

Is this GFM x moles x 1000

#### chenbeier

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« Reply #12 on: November 11, 2020, 03:05:04 PM »
The molarity is given, also the volume. You got the result also between the first calculation.