Gravimetric Analysis- precipitation method

[leah3000]

Homework Statement

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.

Homework Equations

My attempt at a balanced equation for this:

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

not sure if that's correct

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 %

Can someone please help me!

 

[Borek]

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?

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methods

 

[leah3000]

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?

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chemical calculators - buffer calculator, concentration calculator
www.titrations.info - all about titration methods

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.

 

[Borek]

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?

This is correct mass of chlorine.

Also if i use this mass to calculate the percentage of Cl, wouldn't that give me the percentage in AgCl (24.8%)?

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.

So then do i just subtract this from 100 to get the percentage in the hydrated halide (75.2%)?

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

I'm really not sure about this...can u explain the MCl2.2H2O part please.

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?

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[DeeNos]

I would approach this problem by first checking the balanced equation, which is correct. Then, I would use the given masses of the hydrated chloride and the resulting silver chloride to calculate the number of moles of chlorine present in the original compound. This can be done by dividing the mass of silver chloride by its molar mass (108.5 g/mol) and multiplying by the molar ratio between chlorine and silver chloride (1:1).

Next, I would use this number of moles of chlorine to calculate the molar mass of the original hydrated chloride compound. This can be done by dividing the mass of chlorine (from the previous step) by the number of moles of chlorine and then adding the molar mass of water (18 g/mol) for each mole of water in the compound.

Finally, to calculate the percentage of chlorine in the hydrated chloride, I would divide the mass of chlorine (from the previous step) by the molar mass of the original compound and multiply by 100. This will give the percentage of chlorine in the compound.

To find the relative atomic mass of the metal, M, we can use the molar mass of the original compound (calculated in the previous step) and subtract the mass of chlorine (35.5 g/mol) and the mass of water (18 g/mol) for each mole of water in the compound. This will give us the mass of the metal in one mole of the compound, which can then be converted to the relative atomic mass of the metal.

Overall, gravimetric analysis using the precipitation method is a reliable and accurate technique for determining the composition of a compound. However, it is important to carefully consider the stoichiometry of the reaction and to perform multiple trials to ensure accurate results.