I'm really struggling with understand how to dilute liquid reagents. With powder reagents it's easy. You just measure them by mass.
For sulphuric acid, I have 98% concentrated sulphuric acid, molecular weight 98.07. I need a formula that tells me how many milliliters of
concentrated acid I need to get 180g/L
For the hydrochloric acid, it gets much worse. I need to know how many ml of 36% concentrated acid (m.w 36.46) I need, to get 50mg/L chloride ions.entropy51 - 6-5-2011 at 06:42
There are lots of threads on the topic of diluting reagents. In one of them I posted these two helpful links:
This looks like homework to me.
Have a stab at it first and I will help you out.
I can do it in a couple of minutes but you will learn very little by copying my answer.jonco - 6-5-2011 at 09:33
Ok, this is how I calculated it. Check my answers.
1. 98% means 98g/100ml. So 180g/L = 100ml/98g*180g = 183.67ml
2. HCL has equal number of hydrogen and chloride atoms so there are 36/2 = 18% chloride ions. 18% is 18g/100ml, so 50mg/L = 100/18*0.05 = 0.278ml ScienceSquirrel - 6-5-2011 at 09:56
The density of suphuric acid is 1.84 g per ml
Therefore 100ml of sulphuric acid is approximately 98*1.84= 180g of acid and 2g of water.
Therefore 100ml added to 900ml of water will produce a solution of ca 180/l sulphuric acid.jonco - 6-5-2011 at 12:19
Thanks
Do I need to use density of hydrochloric acid in the calculation of the chloride ion, or is my calculation correct for that part?ScienceSquirrel - 6-5-2011 at 14:24
50mg = 0.05g
0.05/35.45 = 0.0014M
100ml of 36% w/v hydrochloric acid = 36/36.45 = 0.987M
0.0014*100/0.987 = 1.42mljonco - 6-5-2011 at 15:13
I don't understand this. Can you explain it?ScienceSquirrel - 6-5-2011 at 15:25
50mg = 0.05g
This converts 50mg in to grams, getting all your units handy is the first move in calculations. Metric units are a lot better than US or Imperial
units in my opinion; grams per litre is a lot easier than ounces per gallon
0.05/35.45 = 0.0014M
Mass of chlorine required per litre divided by the atomic mass gives us the number of moles required per litre.
100ml of 36% w/v hydrochloric acid = 36/36.45 = 0.987M
This calculates the number of moles in 100ml by dividing the grams per 100ml by the molecular mass.
0.0014*100/0.987 = 1.42ml
Number of moles required multiplied by the number of mls that hold a known number of moles gives the number of mls required. jonco - 6-5-2011 at 15:50
Wouldn't it be more accurate to take the density into account?
Density of HCL = 1.19g/ml
1000ml has 1.19g/ml*1000ml = 1190g HCL
36% HCL = 36%*1190g = 428.4g HCL
Moles HCL = 428.4g/36.45g/mol = 11.75 mol
We want 50mg of Cl- which is 0.05/35.45 = 0.0014 mol
There is 11.75 mol per liter Cl- ions in the stock solution.
To get 0.0014mol Cl- ions, we measure out
0.0014*1000/11.75 = 0.12ml of the 36% HCL.
[Edited on 6-5-2011 by jonco]ScienceSquirrel - 6-5-2011 at 16:21
I agree with that.
An adjustment to allow for the density is a good idea.
The acids will not be exactly 98 and 36% so you have to allow for some variation there as well.
I would be pleased if I got 180g/l plus or minus 2 grams and 50mg/litre plus or minus 5mg if I carried out the dilutions in practice using bottles
of reagent grade acid picked off the shelf.
jonco - 6-5-2011 at 16:30
There is quite a big difference between your answer of 1.42ml and my answer of 0.12ml for the chloride ions.
Which is the correct answer?entropy51 - 6-5-2011 at 16:34
100ml of 36% w/v hydrochloric acid = 36/36.45 = 0.987M
My references say conc HCl is 36% w/w with a density
of 1.18 and is 11.6 mole/L. Therefore 100 mL contains 1.16 mole of HCl.
[Edited on 7-5-2011 by entropy51]ScienceSquirrel - 6-5-2011 at 16:55
I did not correct for the density so 1.2ml is closer to the correct answer than 1.42ml.
But the acid will not be exactly 36% per cent even if you have a fresh bottle direct from the supplier.
If we use the same bottle of acid and make up our solutions, my solution would probably be stronger than would be expected but your solution would not
be exactly 50mg/l either.
In practice if you wanted to make up a solution like this you would dilute the hydrochloric acid to about 0.1M. Titrate the solution to get an exact
value and then dilute the solution again.
This would give you a solution of 50mg/litre with an error of less than 1mg/ litre or better.entropy51 - 6-5-2011 at 17:05
Should that be 0.142ml?ScienceSquirrel - 6-5-2011 at 17:30
Oops, I got the decimal point in the wrong place and failed to read your posts closely enough!
If it is a homework question you can take your pick between 0.12 and 0.142 ml depending on what you think the teacher may regard as the correct
answer.
If you actually need to do it then follow a better procedure.
