Help Needed With Percentages Again.

[Annie33]

Hello, I've already posted something similar and don't understand the answer, but now my percentages have changed. Hope someone can help.

Problem: I have some magnesium chloride flake which I want to add to water to make magnesium brine. My magnesium chloride flake contains 47% MgCl. Therefore, if I have 100gram of flake it will contain 47gram of MgCl. If I add 100gram of flake to 100ml of water, I get 200ml of brine containing 47gram of MgCl, but the concentration of MgCl in 200ml of brine is 23.5% (because I diluted the MgCl concentration by half).

I want to make up solutions of brine containing 15% MgCl and 29% MgCl and a third solution containing 31% MgCl.

I think that if I could work out how many grams or miligrams of flake are equal to 1%, then I could multiply that figure by 15, and 29 and 31 consecutively to get my answers ... but I'm not sure how to do this.

Is this correct, please:

47 gram of MgCl = 47,000mg of MgCl
Therefore 47,000 divided by 100 = 470
Therefore 470mg of MgCl = 1%

Therefore 470mg of MgCl flake x 15 = 7,050 mg (?approx 7 grams)
Therefore if I add 7 grams of MgCl flake to 100ml of water will I have approximately 15% MgCl concentration?

 

[guitarguy]

You mostly have this problem correct. One milliliter of water weighs one gram, so you have 200 milligrams of solution. By adding 100 grams of 47% Magnesium flake to 100 grams of water I agree this is a 23.5 % solution.
So if you have X amount of solution you have 0.47*X amount of Magnesium. To find a certain percentage of solution:
magnesium divided by total solution equals percent solution divided by one hundred or
(0.47)*X/(X+100) = percent/100. In this case you are asked to find 15% and other percentages.
Hope this helps.

 

[Annie33]

Don't you need to do a conversion? grams to milliliters?

As an aside, can't you ask your pharmacist?

Hi Denis, someone told me that 1ml of water weighs 1 gram so when trying to work out this calculation I've been flipping between grams and mls thinking that it doesn't matter which measure I use. And thankyou for the tip, a pharmacist might be willing to help.

 

[Annie33]

You mostly have this problem correct. One milliliter of water weighs one gram, so you have 200 milligrams of solution. By adding 100 grams of 47% Magnesium flake to 100 grams of water I agree this is a 23.5 % solution.
So if you have X amount of solution you have 0.47*X amount of Magnesium. To find a certain percentage of solution:
magnesium divided by total solution equals percent solution divided by one hundred or
(0.47)*X/(X+100) = percent/100. In this case you are asked to find 15% and other percentages.
Hope this helps.

Thank you Guitar Guy, I understand now. Simple when you know how!