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Checking my understanding of Molarity.

  1. Sep 13, 2004 #1
    Ok, I just started chemistry last tuesday, and I want to check my understanding of the concept of Molarity.

    Let's use Carbon as an example, which has an atomic weight of 12.0107 atomic mass units. But that's too small to be of any practical use, so whoever figured out that if you have 1 Mole of Carbon, that you'd get an equivalent number of grams, and by knowing how many grams of carbon you have, or whateve element, you can mix exact proportions of different atoms etc.. So one Mole of Carbon would weight 12.0107 grams.

    Is that pretty much it, from the perspective of a kid taking chemistry for 5 days?
  2. jcsd
  3. Sep 13, 2004 #2
    Yes, that's pretty much it.
    Avogadro has given name to the quantity of one mole, which is 6.022E23 atoms or molecules or whatever particle you are counting, but if you are only concerned with moles and molar masses and don't need to know about the individual atoms, you will not need this huge number.
    I've got my lab protocol in front of me and can see that I recently mixed:
    0.499 g K, which is 0.499 g / (39.10 g/mol) = 12.8 mmol of potassium
    0.309 g Ti(OC3H8)4 which is 0.309 / (284.23 g/mol) = 1.09 mmol of titanium
    0.192 g H2O, which is 0.192 / (18.02 g/mol) = 1.06 mmol of water
    1.099 g EuCl3, which is 1.099 / (258.35 g/mol) = 4.26 mmol of europium chloride
    (mmol = millimole: 499 mg / (39.10 mg/mmol) = 12.8 mmol)

    Thus, I've mixed potassium, a titanium compound, water and europium chloride in a molar proportion of about 12:1:1:4. By doing so, I guaranteed that for each water molecule I've added 4 europium chloride units (4 europium ions and 12 chloride ions), but I don't have to use these tiny atomic mass units, or these large molecule counts (1.06 moles of water = 6.38·10^23 water molecules) and life is much easier for me this way.
    Last edited: Sep 13, 2004
  4. Sep 13, 2004 #3


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    What you have described is not molarity, but the concept of moles of a compound and the utility of a molar ratio. At its most basic level, moles are a measure of the amount of molecules in a sample, but since that is not a quantity that is easily measure, the molar mass allows one to relate the number of molecules to a measurable mass. I think that this is essentially what you said, just turned around a bit.

    Molarity is a measure of concentration. It is defined as the number of moles of a substance per liter of solution. So 1 mole of NaOH in 1 L of water is a 1 molar solution of NaOH. Again, this is a means to get a measurable quantity (volume of solution) that relates to the number of molecules of material in the solution (moles).

    I think you have essentially the right idea. I hope that this clarifies the relations a little better.
  5. Sep 13, 2004 #4
    You kind of have the idea of what a mole is. Becareful when you talk about moles, a mole does not mean the same thing as molarity. A mole is just a way really of counting atoms. For example you can have a dozen baseballs and a dozen bb's, but they don't have the same mass. A mole just means you have 6.02x10^23 atoms (avogadro's #) of whatever atom or molecule you are talking about. A mole of an atom has mass that is equal to the atomic mass in g's. You can also thinking of it like baking. When you bake you have to mix things in proportion to one another. Same thing in chemistry, except the units in chemistry aren't cups, tablespoons, etc. but moles.
  6. Sep 13, 2004 #5
    Oh, yes, sorry for contributing to the confusion, I reacted on the contents of the first post and forgot the title question.

    There are two concepts:
    -mole is the one both you and I were talking about. It's a unit of "amount".
    -molarity is the quantity associated with the unit molar (M).

    So what I described was how many moles I had of the named compounds. Actually, these were dissolved in 25 ml of a solvent. As movies said, one molar is one mole per litre. Thus, the molarity of potassium in my solution was 12.8 mmol K / 25 ml = 0.51 M. (Note btw that I corrected the moles of my previous post to millimoles :smile:)
  7. Sep 13, 2004 #6
    And if you are east-asian, you might have problems with the concept molality. Just a quick review in case you encounter that one as well, note that they are not at all analogs.

    One molar is one mole per litre of the total solution. If the density does not change after you prepared your solution, you may take a certain volume of this solution and if you know its molarity in some species/element, you know the amount of that species by n = volume * molarity (13 mmol = 25 ml * 0.51 M).
    One molal is one mole of a substance added to one litre of solvent. But... actually I don't remember anymore what this concept molality is used for... I think it was needed when calculating with properties like boiling point elevation... :confused:
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