Ch.4 - Reactions in Aqueous Solution

McMurry8th EditionChemistryISBN: 9781292336145당신이 사용하는 게 아니라요?교과서 변경

McMurry 8th Edition

Ch.4 - Reactions in Aqueous Solution

문제 155b

4장, 문제 155b

Sodium nitrite, NaNO2, is frequently added to processed meats as a preservative. The amount of nitrite ion in a sample can be determined by acidifying to form nitrous acid (HNO2), letting the nitrous acid react with an excess of iodide ion, and then titrating the I3 - ion that results with thiosulfate solution in the presence of a starch indicator. The unbalanced equations are (1) (2) (b) When a nitrite-containing sample with a mass of 2.935 g was analyzed, 18.77 mL of 0.1500 M Na2S2O3 solution was needed for the reaction. What is the mass percent of NO2- ion in the sample?

검증된 단계별 안내

Identify the relevant chemical reactions involved in the process. The reactions are: \( \text{HNO}_2 + \text{I}^- \rightarrow \text{NO} + \text{I}_3^- + \text{H}_2\text{O} \) and \( \text{I}_3^- + 2\text{S}_2\text{O}_3^{2-} \rightarrow 3\text{I}^- + \text{S}_4\text{O}_6^{2-} \).

Calculate the moles of \( \text{Na}_2\text{S}_2\text{O}_3 \) used in the titration using the formula \( \text{moles} = \text{Molarity} \times \text{Volume} \). Convert the volume from mL to L before using it in the calculation.

Use the stoichiometry of the second reaction to find the moles of \( \text{I}_3^- \) that reacted. According to the balanced equation, 1 mole of \( \text{I}_3^- \) reacts with 2 moles of \( \text{S}_2\text{O}_3^{2-} \).

Use the stoichiometry of the first reaction to find the moles of \( \text{NO}_2^- \) in the sample. According to the balanced equation, 1 mole of \( \text{NO}_2^- \) produces 1 mole of \( \text{I}_3^- \).

Calculate the mass percent of \( \text{NO}_2^- \) in the sample using the formula \( \text{mass percent} = \left( \frac{\text{mass of } \text{NO}_2^-}{\text{mass of sample}} \right) \times 100\). Use the molar mass of \( \text{NO}_2^- \) to convert moles to grams.

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23m

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주요 개념

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Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions. It involves using balanced chemical equations to determine the relationships between the amounts of substances consumed and produced. In this context, stoichiometry is essential for converting the volume and concentration of thiosulfate solution used in the titration into moles, which can then be related back to the amount of nitrite ion present in the sample.

Titration

Titration is a quantitative analytical technique used to determine the concentration of a solute in a solution. It involves the gradual addition of a titrant to a solution until a reaction is complete, indicated by a color change or a specific endpoint. In this case, the titration of I3- with thiosulfate allows for the determination of the amount of nitrite ion in the sample, which is crucial for calculating its mass percent.

Mass Percent

Mass percent is a way of expressing the concentration of a component in a mixture as a percentage of the total mass. It is calculated by dividing the mass of the component by the total mass of the sample and multiplying by 100. In this problem, calculating the mass percent of nitrite ion involves determining the mass of NO2- from the titration results and relating it to the total mass of the processed meat sample.

관련 실천

교과서 질문

Sodium nitrite, NaNO2, is frequently added to processed meats as a preservative. The amount of nitrite ion in a sample can be determined by acidifying to form nitrous acid (HNO2), letting the nitrous acid react with an excess of iodide ion, and then titrating the I3 - ion that results with thiosulfate solution in the presence of a starch indicator. The unbalanced equations are (1) (2) (a) Balance the two redox equations.

To 100.0 mL of a solution that contains 0.120 M Cr(NO3)2 and 0.500 M HNO3 is added to 20.0 mL of 0.250 M K2Cr2O7. The dichromate and chromium(II) ions react to give chromium(III) ions. (a) Write a balanced net ionic equation for the reaction.

477

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교과서 질문

(b) When a brass sample with a mass of 0.544 g was sub-jected to the preceding analysis, 10.82 mL of 0.1220 M sodium thiosulfate was required for the reaction with iodine. What is the mass percent copper in the brass?

385

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교과서 질문

Brass is an approximately 4:1 alloy of copper and zinc, along with small amounts of tin, lead, and iron. The mass per-cents of copper and zinc can be determined by a procedure that begins with dissolving the brass in hot nitric acid. The resulting solution of Cu2+ and Zn2+ ions is then treated with aqueous ammonia to lower its acidity, followed by addi-tion of sodium thiocyanate (NaSCN) and sulfurous acid (H2SO3) to precipitate copper(I) thiocyanate (CuSCN). The solid CuSCN is collected, dissolved in aqueous acid, and treated with potassium iodate (KIO3) to give iodine, which is then titrated with aqueous sodium thiosulfate (Na2S2O3). The filtrate remaining after CuSCN has been removed is neutralized by addition of aqueous ammonia, and a solu-tion of diammonium hydrogen phosphate ((NH4)2HPO4) is added to yield a precipitate of zinc ammonium phosphate (ZnNH4PO4). Heating the precipitate to 900 °C converts it to zinc pyrophosphate (Zn2P2O7), which is weighed. The equations are (1) (2) (3) (4) (5) (a) Balance all equations.

843

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교과서 질문

(b) Calculate the concentrations of all ions in the solution after reaction. Check your concentrations to make sure that the solution is electrically neutral.

847

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