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Thread: Help needed with percentages please

  1. #1

    Help needed with percentages please

    Can anyone help me with a problem, please, that is driving me nutty. Sorry if itís a bit longwinded. I want to start making my own magnesium oil (which is really a brine) because it's cheaper than buying the ready made stuff. I'm trying to get the dosage right for my skin, but I'm struggling with my maths.

    I have bought some magnesium chloride flakes which contain approx 46% magnesium chloride and the rest is water content. I have been told to add equal quantities of flake to warm water which will produce a brine/oil containing 23% magnesium. Therefore if I take 500g of magnesium chloride flake and dissolve it in 500ml of water to get 1 litre of brine, the magnesium content is 23g, or 23,000mg.

    So, if I want to calculate how many mg of magnesium is in 1 ml of brine, I divide 23,000mg by 1000 ml which gives me 23mg of magnesium per ml of brine, which gives me 115mg of magnesium per 5ml teaspoon. Am I right so far, please?

    This is where I have a brain block and get confused: Iíve bought myself an atomizer and Iíve calculated that I can get 80 sprays out of 10ml of the brine; which means (I think) that there are 8 sprays per 1 ml of brine. Therefore if 8 sprays give 1ml of brine, does that mean that there is 2.875mg of magnesium per spray (I did 23mg divided by 8 to get that figure)?

    But 10 sprays would only give me 28.75mg of magnesium which doesn't seem strong enough for me, as I'd have to spray my skin loads of times to get 100mg of magnesium on my skin via the spray bottle; so if I want to double it to start with (to get a solution which is twice as strong) presumably I need to double my magnesium flake content per mls of water? Therefore instead of using 500g of flake per 500ml, should I use 1kg of flake per 500ml of water, to give a 46% brine solution? If so, this would presumably give me 46mg of magnesium per ml of brine, and 230mg of magnesium per 5ml teaspoon; and 5.75mg of magnesium per spray (2.875 x 2)? This still doesn't seem strong enough, for my spray bottle, but I'll come back to this later.

    Also, (this is where I show my ignorance big time) if I make up 1 litre of brine solution which contains 23% magnesium, (made up from 500g of mag flake Ė 46% mag content - plus 500ml of water), then Iíll get a magnesium brine containing 23% magnesium which will contain 230grams of magnesium per litre of brine solution. However, if I siphon off 100ml, will my 100ml bottle of magnesium contain only 23gram of magnesium because 23% of 100 is 23?

    Finally, regarding my need to get a stronger solution: the last magnesium oil product I bought from the shop contains 31% magnesium content per 100ml of brine/oil, and they say that each spray contains 19mg of magnesium (so 10 sprays should give 190mg of magnesium) Ė and they say that their bottle contains up to 800 sprays. So how many grams of my magnesium chloride flake (at 46% mag content) would I need to add to 1 litre of water to get a 31% magnesium content in the brine solution, please?

    Thanks in advance to anyone who might be able to help.

    Annie.
    Last edited by Annie33; 04-02-2012 at 09:42 AM. Reason: Added spaces between paragraphs

  2. #2
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    Quote Originally Posted by Annie33 View Post
    Can anyone help me with a problem, please, ...

    Finally, ... So how many grams of my magnesium chloride flake (at 46% mag content) would I need to add to 1 litre of water to get a 31% magnesium content in the brine solution, please?

    Thanks in advance to anyone who might be able to help.

    Annie.
    1. Let x denotes the amount of MgCl-flakes. Then the content of pure Mg is 0.46 * x. When you add 1 litre of water the total mass is (1 + x). And this mass contains 31% or 0.31 parts of Mg. Both amounts must be equal.

    2. Thus solve for x:

    0.46*x = 0.31 * (1 + x)

    I've got x = 2.0667 kg

    3. That means you have finally a total mass of 3.0667 kg of 31% solution.

  3. #3
    Hi Pappus, Many thanks for your help. I appreciate it. I don't understand how you worked it out but I'll keep reading your answer and hopefully it will make sense. I assume that the asterix * is a multiplication symbol?

    While in bed almost asleep last night I realised I'd made a mistake in my original calculation ... I said in my first paragraph that there were 23g of magnesium in a litre of oil made up from 46% Magnesium chloride flake; but there isn't 23g, there are 230g because 23% of 1000 ml (1 litre) is 230 not 23. I think that's partly what's been confusing me ... I don't think it will affect your calculations.

    Does your answer mean that I need to mix more than 3kg of MgCl-flakes (at a 46% MgCl concentration) to 1 litre of water to get a finished solution containing 31% mgCl content?

  4. #4
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    Quote Originally Posted by Annie33 View Post
    Hi Pappus, Many thanks for your help. I appreciate it. <--- You are welcomed!
    I don't understand how you worked it out but I'll keep reading your answer and hopefully it will make sense. I assume that the asterix * is a multiplication symbol? <--- Yes.

    While in bed almost asleep last night I realised I'd made a mistake in my original calculation ... I said in my first paragraph that there were 23g of magnesium in a litre of oil made up from 46% Magnesium chloride flake; but there isn't 23g, there are 230g because 23% of 1000 ml (1 litre) is 230 not 23. I think that's partly what's been confusing me ... I don't think it will affect your calculations. <--- Correct

    Does your answer mean that I need to mix more than 3kg of MgCl-flakes (at a 46% MgCl concentration) to 1 litre of water to get a finished solution containing 31% mgCl content?
    1. No.

    2. x denotes the amount of MgCl-flakes. According to my result you have to take 2.0667 kg of MgCl-flakes, add 1 kg (= 1 liter) of water. This will give you 3.0667 kg of a ready-to-use solution.

    3. The equation I used in my previous post refers to the amount of effective substance:

    0.46*x = 0.31 * (1 + x)

    0.46x describes the amount of Mg in the flakes,
    0.31(x + 1) describes the amount of Mg after you have diluted the original substance.
    In both cases the amount of Mg must be equal.

  5. #5
    Hi Pappus,

    Again, many thanks for your help. I'm ashamed to say that I don't understand equations. Your calculations look and sound fascinating, I wish I understood them but I don't. I think I'll have to research the internet to find out how to do equations. I can follow your workings out a little bit, but not totally.

    Thanks again, and best wishes to you.

    Annie

  6. #6
    Elite Member
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    Well Annie, why don't you tell us where you're at;
    grade? student attending math classes? .....?

    Then we'll know HOW to help you...
    I'm just an imagination of your figment !

  7. #7
    Quote Originally Posted by Denis View Post
    Well Annie, why don't you tell us where you're at;
    grade? student attending math classes? .....?

    Then we'll know HOW to help you...
    Hi Denis,

    I don't go to any maths classes. I'm 47 years old! I passed my GCSE maths equivalent in my 30s when I went back to college to continue my education. Here in England GCSE maths is the exam that 16 year olds generally take prior to leaving school. As an adult I took an 'equivalent' GCSE maths programme and passed at grade 'C', which I understand is average - but the equivalent exam doesn't give out grades above grade C, so I may have had the ability to pass at Grade A for all I know ... but I don't think so in reality I have a bit of a health problem which is remedied by drinking lots of milk kefir for the probiotics, but my calcium intake has shot up, hence I'm trying to balance it with the correct ratio of magnesium supplements. The magnesium oil I buy from the shops is very expensive so I want to make my own by buying the flake at a cheap rate and mixing with water but I'm struggling to get the ratio that I want.

    I understand in very basic terms the concept of equations - but I couldn't really follow the answer given to me to above. Thanks for offering to help.

    Annie

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