Molarity and molality question

Discussion in 'DAT Discussions' started by SN1, May 31, 2012.

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  1. SN1

    SN1 Banned

    Jan 27, 2011
    Hi I stumbled upon two questions on destroyer and I would appreciate if someone can explain them clearly. I got the correct answer but I would like to know if my understanding and assumption is correct. I am going to be relating both questions back and forth.

    question 1

    A solution of LiCl has a density of 1.13g/cm^3 at 25 degree celsius. If the solution is 20% by weight LiCl, what is the molarity?

    the answer is 5.3 M

    So I know that
    Molarity= # of moles of solute/liters of solution

    Now I must find the number of moles of the solute first
    and since density= mass/volume then 1.13g is the MASS of the SOLUTION PER .001 Liter? am I correct to say this? when I multiply 1.13g by 20% I am finding the MASS of LiCl alone correct?

    so only 20% of those solution is actually LiCl and to find the moles of LiCl I need to find mass of LiCl first. So to find the mass of LiCl I need to do
    .2 * (1.13/.001L) correct?

    then after I do that my answer would be in g/ L and to convert that to moles I would just divide the answer by the molar mass of LiCl and get the correct answer of 5.3moles/liter or Molarity.

    are the things that I said correct?

    Question 2
    what is the molality of a 14% (w/w) glucose solution (molar mass of glucose is 180g/mole)

    Now I know that molality = # of moles of solute / kg solvent

    the answer key automatically said that 14% means 14 g of solute/100 g solution. But how can you say this? is this just an assumption? so for the first question I won't be able to say this because they actually GAVE me the mass of the solution right? so I can't just assume I had 20g of LiCl? and that's why I had to multipy 20% by 1.13g correct?

    Ok the solution then went on and divide 14g/180g/mole of glucose and got # of moles of glucose.

    then after they got the moles of glucose they divide that number by .086 kg of solvent.

    Now the .086 kg comes from the remaining of the solution correct ? which was 86 g of water.

    so 100 g of solution made up of 86g of water and 14 g of glucose correct?

    what I am confused about is that why on the second question I can just assume 14g of glucose while on the first question I had to multiply 20g by 1.13

    is this because on the first question they ACTUALLY gave me the mass of the solution in a form of density which let me calculate the mass of the LiCl itself?

    thanks and sorry for the length
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  3. jaymin8

    jaymin8 2+ Year Member

    Jul 17, 2011
    For the First question:

    Density equals mass/ML!!!!!
    Molarity equals mol/LITERS!

    So convert density so it is over liters.
    density is 1.13g/ML ~ 1130g/L

    Lets assume the entire solution is one liter. That means you have a 1130 gram solution.
    Take 20% of that to get the LiCl.
    That gives 226g.
    Now to get moles you do 226grams/(molar mass~42.35)= 5.3M
  4. jaymin8

    jaymin8 2+ Year Member

    Jul 17, 2011
    Sorry for posting twice. They gave you the density for the first problem so you use that to get the mass. For the second problem you use the assumption because no mass is given. What you said was correct.
  5. SN1

    SN1 Banned

    Jan 27, 2011

    Thanks a lot I feel pretty confident about it now.

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