it's a yes, actually ...

1. Calcium ions can be determined by precipitation of the ethandioate according to the equation:

Ca²⁺(aq) + (COO^-)₂(aq) --> Ca(COO)₂(s)

The solid may then be filtered off, washed and ethandioic acid regenerated by the addition of excess sulfuric acid. The amount of ethandioic acid is then found by titration using potassium manganate(VII) according to the equation:

5H₂C₂O₄ + 2MnO₄^- + 6H⁺ --> 10CO₂ + 2Mn²⁺ + 8H₂O

In an experiment to find the concentration of calcium ions 100cm³ of an unknown calcium hydrogen carbonate solution was reacted with excess sodium ethandioate and the calcium ethandioate precipitate collected, washed and dried.

Excess dilute sulfuric acid was added to this precipitate with stirring until it had all dissolved and the solution made up to 250 cm3 with distilled water in a volumetric (graduated) flask. A 25 cm³ portion of this solution required 10.5 cm³ of 0.02 mol dm^-3 potassium manganate(VII) solution for complete reaction. Calculate the concentration of the original calcium hydrogen carbonate solution.

(edited 7 years ago)

Reply 6

rosemondtan

Is this for AS or A2?

Reply 7

jonnypdot

Original post by charco

thanks i got 0.0525mol per dm3

Reply 8

charco

Study Forum Helper

Original post by jonnypdot

thanks i got 0.0525mol per dm3

is correct :smile:

1. Brass is a copper containing alloy with many uses. An analysis is carried out to determine the percentage of copper present in three identical samples of brass. The reactions involved in this analysis are shown below.

Step 1: Cu(s) + 2HNO₃(aq) + 2H⁺(aq) --> Cu²⁺(aq) + 2NO₂(g) + 2H₂O(l)
Step 2: 4I^-(aq) + 2Cu²⁺(aq) --> 2CuI(s) + I₂(aq)
Step 3: I₂(aq) + 2S₂O₃^2-(aq) --> 2I^-(aq) + S₄O₆^2-(aq)

A student carried out this experiment three times, with three identical small brass nails, and obtained the following results.
Mass of brass = 0.456 g ± 0.001 g
Average volume added of 0.100 mol dm^-3 S₂O₃^2-(aq)(± 0.10 cm³) added = 28.50
Calculate the percentage by mass of copper in the brass.

Reply 9

jonnypdot

Original post by charco

is correct :smile:

39.69% , are you making these up?

Reply 10

Pigster

I'm not very sorry about the formatting. If you don't like it, you know where you can stick it :smile:

This question is about an oxidation state change of ruthenium.
Step 1
14.50 g of iodine is dissolved in a dilute sulfuric acid solution.
4.41 g of ruthenium(III) chloride, RuCl3, is added.
Some of the iodine is reduced and the ruthenium(III) chloride is converted into ruthenic acid.
Step 2
Starch is added to the solution containing ruthenic acid and the excess iodine.
The solution is made up to 250 cm3 using distilled water.
25.0 cm3 of this solution is transferred into a conical flask using a glass pipette and filler.
Step 3
A burette is filled with a 0.200 mol dm–3 solution of sodium thiosulfate, Na2S2O3.
A titration was carried out until consistent values for the titre were obtained.
The equation for the reaction between the excess iodine and sodium thiosulfate is shown below.
I2 + 2S2O32– → 2I– + S4O62–
Sodium thiosulfate does not react with ruthenic acid.
(a) The results for the titration are shown below.
Trial 1 2 3 4
Final burette reading / cm3 29.15 32.45 34.20 27.35 30.65
Initial burette reading / cm3 3.20 6.80 8.85 2.15 5.40
Titre / cm3
(i) Use the consistent values to calculate the mean titre to an appropriate number of significant figures.
mean titre = …………………………. cm3 [1]
(ii) The burette has a maximum error of 0.05 cm3 for each measurement.
Calculate the percentage error in the titre for experiment 4.
percentage error = …………………………. % [1]
(iii) Using the same equipment, suggest a change to the experiment that would reduce the value for the percentage error of experiment 4. ………………………… [1]
(b) The ruthenic acid that forms in step 1 is effectively colourless, so does not affect the colour of the solution.
State the colour of the iodine solution in step 1. ………………………
State the colour of the iodine solution in step 2 after the starch is added. ………………………
State the colour of the solution in step 3 when the end-point of the titration is reached. ……………………… [1]
(c) Calculate the oxidation state of ruthenium in ruthenic acid.Show your working. …….…………………… [7]

Reply 11

charco

Study Forum Helper

Original post by jonnypdot

39.69% , are you making these up?

... is the correct answer :smile:

Dissolved chlorine can be determined by 1) reaction with excess potassium iodide displacing iodine and then 2) determining the iodine liberated by titration using sodium thosulfate solution:

Cl₂ + 2I^- --> I₂ + 2Cl^-
2Na₂S₂O₃ + I₂ --> Na₂S₄O₆ + 2NaI

A 10 cm³ aliquot of chlorine solution was transferred to a conical flask and excess of potassium iodide (aq) added. The mixture was then titrated against standardised 0.05 mol dm^-3 sodium thiosulfate solution, adding starch as an indicator immediately prior to the disappearance of the yellow iodine colour. The average titre was 27.0 cm³. Calculate the molarity of the chlorine solution

(edited 7 years ago)

Reply 12

jonnypdot

Original post by charco

... is the correct answer :smile:

0.0675 mol per dm3 thank you for helping

Reply 13

charco

Study Forum Helper

Original post by jonnypdot

0.0675 mol per dm3 thank you for helping

... is the correct answer :smile:

The data below is from an experiment used to determine the percentage of iron present in a sample of iron ore.

This sample was dissolved in acid and all of the iron was converted to Fe²⁺. The resulting solution was titrated with a standard solution of potassium manganate(VII), KMnO₄. This procedure was carried out three times. In acidic solution, MnO₄^- reacts with Fe²⁺ ions to form Mn²⁺ and Fe³⁺ and the end point is indicated by a slight pink colour.

burette reading /ml = 22.5 ml
mass of iron ore /g = 0.3682
KMnO₄(aq) concentration /mol dm^-3 = 0.02152

Determine the percentage by mass of Fe present in the sample.