Horizen Chemistry Sample Draft

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VCE Chemistry Week 1 - Chemical Analysis (1)
Horizen Education February 8, 2013

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1.1

Review: Gravimetric and Volumetric Analysis
Gravimetric Analysis

Solubilities Always soluble: Usually soluble: Exceptions:

Na+ , K+ , NO3− , CH3 COO− , NH+ 4 Cl− , I− , Br−

i. Sodium carbonate and Silver nitrate

ii. Iron (II) sulfate and Lead (II) nitrate

iii. Sodium nitrate and Nickel sulfate

iv. Potassium hydroxide and Copper (II) nitrate

v. Sodium sulde and Cadmium sulfate

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Write down the ionic chemical equation when the following pairs of solutions are mixed together. (Take note of their solubilities of the products formed) Question 1.

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Remember that all ionic compounds are soluble to an extent; those classied as insoluble really just have extremely low solubility.

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It is assumed knowledge that AgCl(s) and BaSO4 (s) are not soluble. (Must know for U3)

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This form of analysis focuses on measuring the mass of a precipitate to nd the concentration of an ion in a sample. Gravimetric analysis involves forming an ionic precipitate with the ion to be analysed, and then drying and weighing the precipitate to nd the amount of ion in the original sample.

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Finding ion concentration

Step 1: Weigh sample. Step 2: Dissolve sample in distilled water, and then lter the waste solid out. Step 3: Add an excess of solution to precipitate the ions being analysed Step 4: Filter this precipitate Step 5: Wash the precipitate with deionised water to remove any unwanted ions Step 6: Dry the precipitate at 100◦ C Step 7: Weigh Step 8: Repeat 6-7 until at constant mass. Example 1. A certain barium halide exists as the hydrated salt BaX2 · 2 H2 O, where X is the halogen. The barium content of the salt can be determined by gravimetric methods. A sample of the halide (0.2650 g) was dissolved in water (200 cm3) and excess sulfuric acid added. The mixture was then heated and held at boiling for 45 minutes. The precipitate (barium sulfate) was ltered o, washed and dried. Mass of precipitate obtained = 0.2533 g. Determine the identity of X.

Conditions

When designing the experiment there are several considerations. The known solution must be able to create a precipitate with a component of the unknown chemical which can be measured. This precipitate must have: ˆ A known formula so that it can be used in calculations involving the reaction equation. ˆ A low solubility to ensure that the majority of it can be collected by the lter paper when ltering.

This will minimise the amount of error in measurements.

ˆ Be stable when heated so that it can be dried easily.

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ˆ Not form a precipitate with any other ions present in the sample to ensure that measurements are

accurate. As nitrates are soluble in any ionic compound, they are often used in gravimetric analysis.

Possible errors:

ˆ Reaction did not precipitate fully and hence a smaller mass of precipitate was formed ˆ The precipitate was not rinsed or dried properly ˆ Transfer errors led to a smaller amount of precipitate formed ˆ Errors in rinsing the precipitate

A sample of lake water near an industrial chemical plant was being analysed for toxic Barium ions by adding 0.050M Silver Sulfate standard solution and reacting to form a precipitate. Example 2.

a) Predict what would happen to the calculated concentration of Barium ions if: i) The concentration of the AgSO4 standard solution was actually 0.040M. ii) The precipitate was not rinsed properly after ltration. b) What errors could have caused a i) Higher than expected concentration ii) Lower than expected concentration Note:

Percentage Composition

Percentage composition questions involving a chemical reaction seek to determine the amount, in grams, of a certain chemical found in 100g of a sample. The steps to solve these questions are always the same, and they are as follows: a) Find the...
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