Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Gravimetric Analysis- precipitation method

  1. May 12, 2010 #1
    1. The problem statement, all variables and given/known data

    1.046g of the hydrated chloride of a certain metal were dissolved in water and silver chloride was precipitated by the addition of a slight excess of silver nitrate solution. After purification the silver chloride had a mass of 1.231g. Calculate the percentage of chlorine in the hydrate. If it is of the form, MCl2.2H2O, calculate the relative atomic mass of the metal, M.

    2. Relevant equations

    My attempt at a balanced equation for this:

    XCl.xH2O + H2O + AgNO3 ---> AgCl + XNO3

    not sure if that's correct

    3. The attempt at a solution

    no. of mols AgCl produced = Mass/ Mr = 1.231/(108)+(35.5) = 8.58x 10^-3 mols AgCl

    From the equation, I assumed a 1:1 ration between AgCl and the hydrated chloride.
    Hence, no. of mols hydrated chloride = 8.58x10^-3.

    I was trying to calculate the Mr of the hydrated chloride but how do i calculate this if i have 2 variables? The mass of the element X and the no. of mols of water?

    I was thinking i'd need to calculate the Mr and then divide the mass of chlorine by the Mr of the compound...then multiply by 100 to find the %:confused:

    Can someone please help me!
  2. jcsd
  3. May 12, 2010 #2


    User Avatar

    Staff: Mentor

    You are making it harder than it really is. What is mass of chlorine in 1.231g of AgCl? Once you know mass of Cl, calculating its percentage is a breeze.

    Now, how many moles of Cl? How many moles of MCl2? Molar mass of MCl2.2H2O? Molar mass of M?

  4. May 14, 2010 #3

    ok so to calculate the mass of Cl do i still use the no. of mols as 8.58x10^-3? which could then give a mass of 0.305g?

    Also if i use this mass to calculate the percentage of Cl, wouldn't that give me the percentage in AgCl (24.8%)? So then do i just subtract this from 100 to get the percentage in the hydrated halide (75.2%)?

    I'm really not sure about this...can u explain the MCl2.2H2O part please.
  5. May 15, 2010 #4


    User Avatar

    Staff: Mentor

    This is correct mass of chlorine.

    You can as well use this mass to calculate percentage of chloride in the original chloride - you were given the mass, and you can be sure 0.305g is also a mass of chlorides in the original sample. There is no other source of chlorides.

    No, thats completely off and these numbers are unrelated. When it comes to percentages you need to look at each compound separately.

    Try to answer questions I have already asked (in the order they were asked, using answer to the previous one to find answer to the next). How many moles of Cl? How many moles of MCl2? Molar mass of MCl2.2H2O? Molar mass of M?

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook