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Homework Help: Guys i need help in some chemistry mole exercises please

  1. Jan 21, 2017 #1
    • Please post this type of questions in HW section using the template and showing your work.
    1)When Mg is burned in air,a mixture of MgO and Mg3N2 is produced.If 3,6g of Mg produces 5,7g of
    mixture,what is the percentage by mass of each compound in the mixture?
    2)A solution of an acid having the formula H3NSO3 is prepared by dissolving 9,7g in water and making up the solution to 250ml.On titrating this solution against 0.500M potassium hydroxide,is found that 20ml alkali,requires 25ml aid for neutralisation.Determine the basicity of the acid.
    I really can't understand what do i have to do in those problems of moles.Thank you in advance for your help!
  2. jcsd
  3. Jan 21, 2017 #2


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    Science Advisor
    Homework Helper
    2017 Award

    Hi Giuseppe, :welcome:

    Here at PF we try to help folks help themselves. That's why the homework fora have a template:
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    Fill in especially 2 and 3 -- it's required (see guidelines)
  4. Jan 21, 2017 #3
    well thank you for the answer.Regarding the first one i wrote the formula as:5Mg+O2+2N2=2MgO+Mg3N2 and if 3,6g is Mg and 5,7 is MgO+Mg3N2 i did 5,7-3,6=2,1 to find out the mass of O2+N2(maybe)...but i don t know hwo to find the masses separately.
    Regarding the second one,i can't understad the part of alkali where there are the numbers 20 and 25 ml,and i can't find out the chemical reaction neither...if only i understand these i will maybe be able to solve it alone.Thank you again for your answer,hope you will reply me.
  5. Jan 21, 2017 #4


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    Staff: Mentor

    1. Assume you have x grams of MgO and y grams of Mg3N2. Can you express the mass of the mixture in terms of x, y? Can you express mass of Mg present in both substances in terms of x, y and relevant molar masses?

    2. You can't start with the reaction equation, as it is exactly what you are asked to find. However, you can easily calculate number of moles of the base used and number of moles of the acid present. What does the ratio of the two tell?
  6. Jan 21, 2017 #5
    Thanks for the answer,can the ratio of the second one be H3NSO3:KOH=2:1?that's what i've found out...by the way i can t still understand the first one :)
  7. Jan 21, 2017 #6


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    Staff: Mentor

    Show how, that's not what I got.

    Show what you did.

    In general: "show what you did" is a key to the help at PF.
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