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Homework Help: Molarity of a solution.

  1. Aug 8, 2008 #1
    I have problem and can not solve it:

    A piece of metallic aluminum weighing 2.50 grams is treated with 75.0 mL of sulphuric acid ( d = 1.18 g/cm3 , 24.7% H2SO4 ). After the metal is dissolved, the solution is diluted to 400mL.

    a) Calculate the morality of the solution with respect to the aluminum salt formed.
    b) Calculate the morality of the resulting solution with respect to unreacted sulphuric acid.

    I made a slight attempt, but got lost after it:

    75ml x 1.18g = 88.5g

    Xg / 88.5g = .247
    Xg = 21.859g of H2SO4

    I actually don't even know what I am doing anymore :(
    I'm seeking a learning experience so that I can solve other problems as well. Please, any help would be greatly appreciated.
  2. jcsd
  3. Aug 10, 2008 #2
    Im not sure what the morality of the solution would be...but i may be able to help with the molarity...

    You have 75mL of 24.7% H2SO4. 75 X 0.247 = 18.53mL of pure H2SO4. 18.53mL X 1.18g/cm3 (density) gives 21.86g H2SO4 X 1mol / 98.078g (molecular weight of H2SO4) = 0.2229mol of H2SO4

    Now we have to determine the #moles of aluminum. Assuming Aluminum is purely elemental aluminum, the molecular weight x weight given is 1 mol / 26.98g X 2.50g Al = 0.09266 mol AL

    First thing to do in any problem like this is to convert everything to # of moles.

    We have 0.2229mol H2SO4 and 0.09266mol Al

    The rest is super easy

    for part A. you know the number of moles, and amount of solution (0.400L) so 0.09266mol AL / 0.400L = 0.232M

    for part B. The amount of H2SO4 remaining (assuming all Al reacted) is 0.2229-0.09266 = 0.13024mol of unreacted H2SO4. so 0.13024/ 0.400L = 0.326M H2SO4.

    Hope that helps (and is correct)
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