Chapter 22: Problem 1

Calculate the salinity of sea water (simply expressed as a \% (w/w) of NaCl in water) from the following experimental data: sea water \((10.00 \mathrm{~mL})\) was transferred to a tall beaker, and concentrated nitric acid \((0.5 \mathrm{~mL})\) and deionised water \((200 \mathrm{~mL})\) added. Silver nitrate solution (0.1 M) was added drop-wise from a burette with constant stirring until coagulation of the precipitate began to occur. The suspension was heated to boiling and stirred (5 min) and a few drops of silver nitrate solution added to confirm completeness of precipitation. The suspension was allowed to cool in the dark (30 min) and then filtered through a pre-weighed sintered crucible (porosity 4 ) using dilute solution of nitric acid [3 \(\mathrm{mL}\) of \(2 \mathrm{M}\) dissolved in deionised water \((300 \mathrm{~mL})]\) for transfer and washing. The precipitate and crucible were dried to constant weight to yield silver chloride \((0.8329 \mathrm{~g})\).

Short Answer

Expert verified

The salinity of the seawater is calculated by setting up the equations described in the steps and solving based on the ratios of the masses and molar masses of sodium chloride, silver chloride, and chloride ions, as well as the volume of seawater sample used. Exact value of the % salinity will depend on the numerical solutions of the steps.

Step by step solution

Determine the amount of chloride ions in AgCl

Given that we have 0.8329 g of AgCl after precipitate. The amount of chloride ions can be calculated using stoichiometry of the reaction and by the molar mass of the species. Considering one mole of AgCl gives one mole of Cl-, we can calculate the amount of Cl- using this method: Moles of Cl- = \frac{mass_{AgCl}}{Molar\ mass_{AgCl}} = \frac{0.8329 g}{143.32 g/mol}.

Convert amount of chloride ions to amount of NaCl

Now, we will use the assumption that all chloride ions originated from sodium chloride. Therefore, the moles of NaCl can be determined as the same as the moles of Cl-. Moles of NaCl = Moles of Cl-. Using the quantity of moles of Cl- calculated in the previous step, we can convert this amount to gram of NaCl: mass_{NaCl} = Moles of NaCl * Molar mass_{NaCl}.