I am having trouble with a lab from my chem class

Procedure
Using the analytical balance, weigh out about 0.2 grams of the unknown
Chloride into a clean Erlenmeyer flask. Dissolve the sample in about 100 ml of deionized
Water and add 3 drops of the K2Cr04 solution. Obtain about 100 ml of the
Standard AgN03 Clean a 50 ml buret and fill it
With the standard solution.
Begin to add the AgN03 solution to the dissolved chloride sample. The
Formation of the white precipitate AgCI will cause the yellow solution to take on a
Chalky yellow appearance. Begin to add the AgN03 solution drop by
Drop until the reddish brown color persists throughout the solution upon swirling,
Indicating the completion of the reaction.

This is the worksheet we are filling out.

Sample weight- sample one .2153 sample 2 .2023
Initial buret reading 0.00 0.00
Final buret reading 28.15 27.95
Volume of standard solution
Moles of AgNO3
Moles of Cl- in sample
Grams of Cl- in sample
Percentage Cl- in sample

Average percentage Cl- in unknown

Average deviation












I do not understand how to get the answers for the blank ones. If anyone could help me out that would be great. I would like it explained though not just the answers because I need to know how to do it in the future.

I guess your weighted samples are the same amount, so, we take the average of the two.

Mass of sample = (0.2153 + 0.2023)/2 = 0.2088 g

We take the average of the volume used:

Volume of AgNO3 used = (28.15 + 27.95)/2 = 28.05 cm^3

You must have been given the concentration of the AgNO3.

Number of moles of AgNO3 = 28.05 \times \frac{concentration\, in \,moldm^{-3}}{1000}

This is simple proportions.
1 dm^3 = 1000 cm^3 = concentration
28.05 cm^3 = (concentration/1000) * 28.05

Ok, now, you need the equation for the reaction:

AgNO_3 + Cl^- \rightarrow AgCl + NO_3\.^-

As you see, one mole of AgNO3 reacts with one mole of Cl-. You know the number of moles of AgNO3 from the previous question, use that to find the number of moles of Cl-.

Usually, you pipette the dissolved sample and then you titrate... Since you know the number of moles of Cl- in the titrated dissolved sample, you can find the number of moles in the total amount of dissolved sample. Then, since you know the total number of moles of Cl-, you can find their mass.

Hint: one mole of ion has a mass equal to the relative atomic mass of the substance.

With that mass, you can find the percentage by mass of the Cl- in the sample.

I hope it helped! :)