Chemists use titrations to determine unknown concentrations.
Zinc chloride (ZnCl2) dissolves completely in water with each molecule of it yielding two chloride ions. Chemists can use silver nitrate and a procedure called titration to measure the number of chloride ions that are in a solution, and thus determine the concentration of the zinc chloride. Silver and chloride ions react in a 1 to 1 ratio; since each molecule of zinc chloride releases two chloride ions, the number of zinc chloride molecules present in the solution will be half the number of silver nitrate molecules required to titrate it.
Instructions
1. Measure 2.55 grams of silver nitrate on your balance and dissolve it in 300 mL of water in your 500 mL beaker. Stir the mixture until the silver nitrate dissolves completely. This will create a 0.05 Molar (M) silver nitrate solution that you will use to titrate the zinc chloride solution.
2. Fill your burette with the silver nitrate solution that you just made.
3. **Handle zinc chloride underneath a fume hood**
Put 30 mL of your unknown zinc chloride solution and 3 drops of indicator solution to your 100 mL beaker. Then place the beaker underneath the burette.
4. Twist the valve on your burette to allow a slow stream of silver nitrate to flow into the zinc chloride solution. Swirl the zinc chloride as you add the silver nitrate and close the valve on the burette as soon as you see a faint peach color appear and stay in the zinc chloride. This color means that all of the chloride ions have been bonded to silver ions and the titration is complete.
5. Multiply the number of liters used to achieve the peach color by the concentration of the silver nitrate to obtain the number of moles used to reach the equivalence point. For example, suppose you used 9 mL (0.009 L) of silver nitrate to reach equivalence.
Moles of silver nitrate = 0.009 L x 0.05 moles/L = 0.00045 moles of silver nitrate
Since silver nitrate reacts with chloride ions in a 1 to 1 ratio, we know that there are 0.00045 moles of chloride in the solution. And since each mole of zinc chloride will release 2 moles of chloride ions when it is dissolved, we know that the 30 mL sample of zinc chloride solution contains 0.000225 moles of zinc chloride, half the amount of chloride ions.
6. Divide the number of moles of zinc chloride present in the 30 mL sample by the volume of the sample to calculate the molarity of the unknown zinc chloride solution. In the example:
Molarity of zinc chloride = 0.000225 moles / 0.3 L = 0.0075 M
The concentration of the unknown zinc chloride solution would be 0.0075 M.
Tags: silver nitrate, zinc chloride, chloride ions, chloride solution, zinc chloride, zinc chloride solution, unknown zinc
