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jamgym913 · Sep 23, 2008, 05:37 PM

I have to do a prelab for my lab tomorrow. We are doing a precipitation titration to determine formula masses of an unknown halide. I have some questions, and it would be great if you could help!

1.) If a .372g sample of pure salt presented by the formula MX is known to contain .00246 mole of a Halide ion (X-), calculate the formula mass of the salt.

2.) A student titrated 10.0 ml of solution of an unknown alkali metal halide (0.100 g/10.0 ml) with AgNO3 solution, how many moles of Ag+ ions were required in each sample?

How many moles of halide ion are present in each sample?

Calculate the formula mass for each sample of the unknown salt.

Using following data:

Mass of Alkali Metal Halide- .100g
Volume of .100 M AgNo3 solution used : 13.74 ml

Unknown008 · Sep 24, 2008, 04:27 AM

1) You need to know what does formula mass means. That means the mass of the substance for onle mole of that substance. How many moles of the substance were you given? 0.00246 is the number of moles since the given formula is MX, meaning that the ratio between the two components is 1:1.

Therefore, you have the mass for 0.00246 mol. By proportion, find the mass for 1 mol.

2)

Originally Posted by jamgym

Volume of .100 M AgNo3 solution used : 13.74 ml

You mean 0.100 mol/10.0ml? If so, you have to work it out. You have the volume, 13.74 ml, find the number of moles in that volume. The number of moles of Ag+ will be equal to the answer you got. That answer should also be equal to the number of halide ions in the unknown salt. Then, using the same procedure as in 1), calculate by proportion, the formula mass of the unknown salt.

Hope it helped. Post back if you don't understand something.

jamgym913 · Sep 26, 2008, 08:35 PM

Thank You Very much!! That helped a lot :)

Unknown008 · Sep 27, 2008, 02:24 AM

You're most welcomed!! I hope to see you again sometime!;) If you have more problems, don't hesitate to ask!