YEAR 12 HOMEWORK

SHEET 4 - Mole Theory, Precipitation & GRAVIMETRIC ANALYSIS

Read pp 17 - 28 of Chemistry 2 and run the program "Isotopes" on the Scott Visual Chemistry Program for background on RAM & RIM

A good resource is the online chemistry course at: http:// tqd.advanced.org/3310/lographics/index.html- esp Unit 1, Section 1 (Significant Figures), Section 3 (Formulas), Section 4 (Naming), Section 5 (The Mole and it's uses), Section 6 (Reactions) and Unit 2 Section1 (Precipitation reactions).

For these questions you must need to remember n=m/M, n=cV and pV=nRT and the table of solubilities.

Q1. How many significant figures are in the following

(a) 123 (b) 1230 (c) 1.230 (d) 1000 (e) 0.0001

Q2. Calculate the RAM of element "X" if it has 3 natural isotopes: the first has a RIM of 31.97 and an abundance of 95.1%, the second has a RIM of 32.97 and abundance of 0.70% and the last has a RIM of 33.97 and an abundance of 4.2%. What is the element?

Q3. Calculate the molar mass and percentage compositions of the following compounds:

(a) H₂O (b) CuSO₄ (c) H₂SO₄ (d) C₆H₁₂O₆

Q4. A 0.8361g sample of an organic compound is analysed and found to contain 0.4565g of carbon and 0.0761g of Hydrogen, with the rest of the mass being made up of oxygen. The molar mass of the compound is 88 g/mol. What is the molecular formula of the compound?

Q5. Calculate the amount of substance of:

(a) 30.0g of water (b) 51.0g of ammonia

(c) 5kg of iron (III) oxide (d) 25.0 ml of 0.05 M HCl solution

(e) 3.6L of 0.56M NaNO₃ solution (f) 1.2 x 10²² molecules of methane

(g) 2.0 L of carbon dioxide gas at STP (h) 300 ml of hydrogen gas at 40 ^oC and 90 kPa

Q6. Calculate the mass of

(a) 1.26 mol of V₂O₅ (b) 0.05 mol of K₂Cr₂O₇

(c) 1.65 mol of ethene (d) Al(NO₃)₃ in 50.0 ml of 0.50 mol solution

(e) 1.0 x 10^-3 mol of Platinum (f) 4.5 x 10²⁴ molecules of SO₂

(g) 3 L of Neon gas at 0 ^oC and 760 mmHg (h) 25 ml of Xenon at 1.4 atm and 30 ^oC

Q7 Write the chemical formula for the following compounds:

(a) Sodium carbonate (b) Iron (II) iodide (c) Barium Sulphate (d) Aluminium Sulphate

(e) Tin (II) chloride (f) Chromium (III) fluoride (g) Calcium oxide (h) Silver Nitrate

(i) Carbon dioxide (j) Magnesium chloride (k) Copper (II) acetate (l) ethanol

(m) Sodium dichromate (n) Calcium hydroxide (o) potassium nitrate (q) Lead (II) chloride

(r) Iron(III) carbonate (s) Aluminium oxide (t) Potassium hydrogen carbonate

(u) Silver nitride (v) Calcium phosphate (w) Lithium oxide (x) copper (II) sulphate

(y) copper (II) hydrogen sulphate (z) Zinc dihydrogen phosphate.

Q8. Which of the compounds in Q7 are insoluble in water?

Q9 Write balanced equations (including states) for the following reactions

(a) When an electric current is passed through water, it decomposes into it's constituent elements

(b) A dilute solution of hydrochloric acid dissolves a strip of magnesium to produce hydrogen gas and magnesium chloride

(c) When solutions of potassium iodide and lead (II) nitrate are mixed, a precipitate forms

(d) When carbon dioxide is bubbled through calcium hydroxide solution, calcium carbonate precipitates out of solution

(e) Butane is burned in a plentiful supply of air

(f) Strontium iodide and lithium sulfate solutions are mixed together.

(g) Lead nitrate solution is added to potassium sulfate sulution

Q10. Calculate the mass of magnesium that is burned in excess oxygen when 5.4g of magnesium oxide is produced.

Q11. Aluminium used to be used in flashbulbs to produce a bright flash of light when it is burned in oxygen to form aluminium oxide.

(a) What mass of aluminium oxide is formed from 6.0 g of powdered aluminium

(b) What volume of oxygen is consumed at 20 ^oC and 1.5 atm?

Q12. Silicon carbide is used to coat drill bits to make them wear resistant. It is produced by heating carbon and silicon dioxide in electric ovens. How much silicon carbide would be produced if 1Kg of carbon and 1 Kg of silicon dioxide were reacted according to the equation

3C_(s) + SiO_{(s) ®} SiC_(s) + 2CO₂?

Q13. The sulfur content of coal can be calculated by combusting the coal with solid sodium carbonate and then forming a barium sulfate precipitate. What is the percentage by mass of sulfur in the coal if a 5.00 g sample of coal produces 0.879g of barium sulfate.

Q14. Write a flow chart for a possible gravimetric analysis of the salt content in a packet of salt & vinegar potato crisps. Assume that the analysis can be performed by precipitating the chloride ions as silver chloride.

Q15. The moisture in soil can be determined by heating it at 105 ^oC until constant mass is obtained. In one experiment the following results were obtained.

Mass of crucible 23.45g

Mass of crucible and soil sample 42.16g

Mass of crucible and dried soil 39.89g

Calculate the percentage of water in the soil.

Q16. In order to analyse a silver alloy for it's silver content, 1.25g of the alloy was dissolved in concentrated nitric acid (which converted the silver into silver nitrate). Excess sodium chloride solution was used to precipitate out the silver ions. The mass of the dried precipitate was 1.12g.

(a) Write an equation for the precipitation reaction

(b) Determine the percentage of silver (by mass) in the alloy

(c) Why was an excess of sodium chloride used?

Q17. In gravimetric analyses, what would be the effect of these errors on the final result?

(a) The precipitate is not dried completely before the final weighing

(b) The precipitate is dried before it is washed with distilled water

(c) Some of the original sample was spilt after weighing it.