The standardization of solutions can be done either directly or indirectly. Describe the two methods and list at least two examples of primary standard reagents.

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It is strongly advised that all apparatuses in the set up be disconnected and cleaned thoroughly. Why?

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Record your results in a tabular form. Qualitative Analysis is a process involving the use of bench reagents in the identification of components of a chemical substance. Qualitative analysis involves identifying substances present in a sample qualitatively. It is used to determine the presence of anions and cations in a given sample. However, teachers, tutors and laboratory technicians face several challenges preparing bench reagents needed for qualitative analysis practical works. A Specific Objective In this section, the teacher, the tutor and the laboratory technician should be able to prepare necessary bench reagents for carrying out chemistry experiments to determine unknown cations and anions from given samples. Key Issues Apparatuses Needed by the Teacher, the Tutor or the Laboratory Technician • A Chemical balance or a digital balance • Volumetric flasks • Beakers, watch glasses • Filter papers • Reagent bottles (clear and brown) • Glass rods • Wash bottles • Labels. Apparatuses Needed by the Candidate • Boiling tubes (pyrex) • Wash bottle • A test tube holder • A nichrome or a platinum wire • Watch glasses Note: Each candidate should be given a Qualitative Analysis Guide (QAG) sheet which is provided by the National Examinations Council of Tanzania (NECTA). The guide is easily downloaded at https://www.necta.go.tz/approved_exam_guides. Make copies of the QAG to be enough for all of your candidates. Chemicals • Hydrochloric acid • nitric acid • sulphuric acid • sodium hydroxide

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The titration of 25.0 cm$^3$ of an acid solution gave a result that the molarity of the acid was found to be 0.0833 M. However, the expected molarity of this acid solution was 0.1 M. This indicates that the acid solution is less concentrated than expected. To standardize such a solution, more stock solution needs to be added to it to raise the concentration. The relation of standardization is given by: $$M_{1}V_{1} = M_{2}V_{2} + M_{3}V_{3}$$ Where: $M_{1}$ = required molarity of a solution (known) $M_{2}$ = calculated molarity of a solution (known from the titration experiment) $M_{3}$ = molarity of a stock solution (known) $V_{3}$ = volume of a stock solution (unknown) $V_{1}$ = total volume of a prepared solution for the whole experiment. (a) Calculate the molarity of a prepared solution ($M_2$) if $M_1 = 0.1 M$, $V_1 = 5.0 L$, and the calculated molarity $M_2 = 0.0833 M$ was obtained by titrating 25.0 cm$^3$ of acid with 30.0 cm$^3$ of 0.1 M NaOH. (b) Calculate the volume required for preparing the solution ($V_1$). (c) Calculate the volume of a stock solution required ($V_3$) for the prepared solution if $M_3 = 12 M$.

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The experiment described below is about the effect of concentration of reactants on the rate of reaction. (i) Measure 50 cm3 of 0.1 M sodium thiosulphate ($Na_2S_2O_3$) solution in a beaker. (ii) Draw a cross on a white piece of paper and place the beaker or a conical flask on it. (iii) Add 50 cm3 of the sulphuric acid and immediately start a stopwatch. The flask should be swirled once or twice before it is placed on the piece of paper. (iv) Look down vertically on the cross and record the time taken for the cross to disappear. (v) Repeat procedures (i) – (iv) using 40, 30, 20 and 10 cm3 of sodium thiosulphate. Always top up the solution with distilled water to make 50 cm3 before adding the acid solution. (vi) Tabulate the data from each experiment in columns indicating the volume of sodium thiosulphate solution (cm3), the volume of water (cm3), time (s), and the rate ($s^{-1}$) as shown in Table 10.

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The experiment described below is about the effect of concentration of reactants on the rate of reaction. (i) Measure 50 cm3 of 0.1 M sodium thiosulphate ($Na_2S_2O_3$) solution in a beaker. (ii) Draw a cross on a white piece of paper and place the beaker or a conical flask on it. (iii) Add 50 cm3 of the sulphuric acid and immediately start a stopwatch. The flask should be swirled once or twice before it is placed on the piece of paper. (iv) Look down vertically on the cross and record the time taken for the cross to disappear. (v) Repeat procedures (i) – (iv) using 40, 30, 20 and 10 cm3 of sodium thiosulphate. Always top up the solution with distilled water to make 50 cm3 before adding the acid solution. (vi) Tabulate the data from each experiment in columns indicating the volume of sodium thiosulphate solution (cm3), the volume of water (cm3), time (s), and the rate ($s^{-1}$) as shown in Table 10.

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The diagram below shows a setup for determining the relative atomic mass of an unknown element in a solution by volumetric analysis.

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The calculations here are based on the volume strength of $H_2O_2$ given on Winchester bottle as assay volume strength (% v/v), For example, 100% v/v. From the decomposition equation of $H_2O_2$ to produce $O_2$, From, $2H_2O_2 \rightarrow 2H_2O + O_2$ $2 M H_2O_2 \rightarrow 1 M O_2 = 22.4$ Litres or $1 M H_2O_2 \rightarrow \frac{1}{2} M O_2 = 11.2$ Litres $100\% v/v$ is equivalent to $1 M = \frac{100}{22.4} = 4.46$ Molar. Molarity of stock solution = $\frac{\text{Volume strength}}{11.2}$ Molarity of stock solution = $\frac{100}{11.2} \approx 8.93$ M For example, to prepare 1 litre of 0.1 M $H_2O_2$ solution, Using the dilution law $M_c V_c = M_d V_d$ Where, $M_c$ = molarity of the $H_2O_2$ before the dilution, $M_d$ = molarity required after the dilution (i.e. 0.1 M), $V_c$ = volume of $H_2O_2$ to be drawn from winchester bottle before the dilution, $V_d$ = the resulting volume after the dilution (i.e. 1000 cm3). $V_c = \frac{M_d \times V_d}{M_c}$ $V_c = \frac{0.1M \times 1000 cm^3}{8.93M} = 11.198 \approx 11.2 cm^3$ The volume of the concentrated $H_2O_2$ that should be diluted to 1000 cm3 = 11.2 cm3.

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The experiment described below is about the effect of concentration of reactants on the rate of reaction. (i) Measure 50 cm3 of 0.1 M sodium thiosulphate ($Na_2S_2O_3$) solution in a beaker. (ii) Draw a cross on a white piece of paper and place the beaker or a conical flask on it. (iii) Add 50 cm3 of the sulphuric acid and immediately start a stopwatch. The flask should be swirled once or twice before it is placed on the piece of paper. (iv) Look down vertically on the cross and record the time taken for the cross to disappear. (v) Repeat procedures (i) – (iv) using 40, 30, 20 and 10 cm3 of sodium thiosulphate. Always top up the solution with distilled water to make 50 cm3 before adding the acid solution. (vi) Tabulate the data from each experiment in columns indicating the volume of sodium thiosulphate solution (cm3), the volume of water (cm3), time (s), and the rate ($s^{-1}$) as shown in Table 10.

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The experiment described below is about the effect of concentration of reactants on the rate of reaction. (i) Measure 50 cm3 of 0.1 M sodium thiosulphate ($Na_2S_2O_3$) solution in a beaker. (ii) Draw a cross on a white piece of paper and place the beaker or a conical flask on it. (iii) Add 50 cm3 of the sulphuric acid and immediately start a stopwatch. The flask should be swirled once or twice before it is placed on the piece of paper. (iv) Look down vertically on the cross and record the time taken for the cross to disappear. (v) Repeat procedures (i) – (iv) using 40, 30, 20 and 10 cm3 of sodium thiosulphate. Always top up the solution with distilled water to make 50 cm3 before adding the acid solution. (vi) Tabulate the data from each experiment in columns indicating the volume of sodium thiosulphate solution (cm3), the volume of water (cm3), time (s), and the rate ($s^{-1}$) as shown in Table 10.

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Which of the following best describes the storage of chemicals in a laboratory?

What is the primary hazard associated with handling concentrated acids in a laboratory?

In case of a chemical spill in the laboratory, the immediate first aid action should be:

Table 1 shows the preparation of some common laboratory reagents.

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