Copper can be determined gravimetrically by precipitation as copper(I)thiocyanate, CuSCN. A wrought aluminum-copper sample(4.5167 g) was dissolved in a minimum amount of concentrated HCl followd by the additon of 25 ml of sulfurous acid to reduced the copper. This solution was then diluted with 100ml of deionized water, heated to near boiling and an excess of 10% ammonium thiocyanate, NH4SCN, solution was added. The Precipitate was allowd to settle overnight, then filtered trough a pre-weighed porcelain filter, washed an dried to constan weight at 110 C. The mass of the empty crucible as 23.7157 g and the mass of the crucible with the CuSCN precipitated was 24.0664g.
a) If the maximum % Cu in the sample could be no more than the % Cu in pure CuCl2, calculate the maximun possible mass of Cu in the sample.
b) If you needed to make up a total volume of 100ml of the 10% ammonium thiocyanate solution, give a detailed description of how you would make up this solution in the lab including both the calculations and the lab procedure. NH4SCN is a solid
c) Assuming the maximum amount of Cu in the sample (as calculated in a), calculate the volume of the ammonium thiocyanate solution you would need to add with a 10% excess. Assume an ammonium thiocyanate solution density of 1.00g/ml
d) Calculate the % Cu in the sample given in the problem
a) To determine the maximum possible mass of Cu in the sample, we need to calculate the mass of CuSCN precipitated.
Mass of crucible with CuSCN precipitated = 24.0664 g
Mass of empty crucible = 23.7157 g
Mass of CuSCN precipitated = Mass of crucible with CuSCN precipitated - Mass of empty crucible
= 24.0664 g - 23.7157 g
= 0.3507 g
As the CuSCN formed from the Cu in the sample, the mass of Cu in the sample is equal to the mass of CuSCN precipitated.
Therefore, the maximum possible mass of Cu in the sample is 0.3507 g.
b) To make up 100 ml of a 10% ammonium thiocyanate solution, we need to calculate the mass of ammonium thiocyanate required and the procedure is as follows:
1) Determine the mass of ammonium thiocyanate needed:
Mass of ammonium thiocyanate = Volume of solution (in ml) * Concentration(in g/ml)
Mass of ammonium thiocyanate = 100 ml * (10 g/100 ml)
Mass of ammonium thiocyanate = 10 g
2) Weigh out 10 g of solid ammonium thiocyanate using an analytical balance.
3) Transfer the solid ammonium thiocyanate into a 100 ml volumetric flask.
4) Add distilled water to the volumetric flask until the volume reaches the 100 ml mark. Use a dropper or a pipette to add water slowly until the desired volume is reached.
5) Mix the solution thoroughly by swirling the flask gently.
c) To calculate the volume of ammonium thiocyanate solution needed to add with a 10% excess, we can use the mass of Copper (as calculated in part a).
Mass of Cu = 0.3507 g
Assuming the density of the ammonium thiocyanate solution is 1.00 g/ml, the volume of solution needed can be calculated as:
Volume of ammonium thiocyanate solution = Mass of Cu * (1 / Concentration)
= 0.3507 g * (1 / 0.1 g/ml)
= 3.507 ml
Adding a 10% excess:
Additional volume needed = Volume of ammonium thiocyanate solution * 0.1
= 3.507 ml * 0.1
= 0.3507 ml
Total volume of ammonium thiocyanate solution needed = Volume of ammonium thiocyanate solution + Additional volume needed
= 3.507 ml + 0.3507 ml
= 3.8577 ml
Therefore, you would need to add approximately 3.8577 ml of the 10% ammonium thiocyanate solution.
d) To calculate the % Cu in the sample, we can use the mass of Cu found in part a.
Mass of Cu = 0.3507 g
Mass of sample = 4.5167 g
% Cu in the sample = (Mass of Cu / Mass of sample) * 100
= (0.3507 g / 4.5167 g) * 100
= 7.76 %
Therefore, the % Cu in the sample is approximately 7.76%.
a) To calculate the maximum possible mass of Cu in the sample, we need to determine the mass of CuSCN precipitated.
Step 1: Calculate the mass of the crucible (empty crucible mass - mass of CuSCN precipitated)
Empty crucible mass = 23.7157 g
Crucible mass with CuSCN precipitated = 24.0664 g
Mass of CuSCN precipitated = Crucible mass with CuSCN - Empty crucible mass
Mass of CuSCN precipitated = 24.0664 g - 23.7157 g = 0.3507 g
Step 2: Calculate the mass of copper in the CuSCN precipitated.
Since the molecular weight of CuSCN is 114.61 g/mol, we can calculate the mass of copper using its ratio in the compound.
1 mol of CuSCN contains 1 mol of copper (Cu)
Mass of copper = Mass of CuSCN precipitated * (Atomic mass of Cu / Molecular weight of CuSCN)
Mass of copper = 0.3507 g * (63.55 g/mol / 114.61 g/mol)
Mass of copper = 0.1923 g
Therefore, the maximum possible mass of Cu in the sample is 0.1923 g.
b) To make 100 ml of a 10% ammonium thiocyanate (NH4SCN) solution:
Step 1: Determine the amount of NH4SCN required.
% concentration = (Mass of solute / Volume of solution) * 100
10% = (Mass of NH4SCN / 100 ml) * 100
Mass of NH4SCN = 10% * 100 ml
Mass of NH4SCN = 10 g
Step 2: Dissolve the NH4SCN solid in a small volume of deionized water.
Weigh out 10 g of NH4SCN and dissolve it in a small volume of deionized water in a beaker or flask.
Step 3: Transfer the dissolved NH4SCN solution to a 100 ml volumetric flask.
Use a funnel to carefully transfer the solution to a 100 ml volumetric flask. Rinse the beaker or flask with deionized water, and add enough water to reach the calibration mark on the volumetric flask.
Step 4: Mix the solution.
Close the volumetric flask with a stopper or cap and invert it several times to ensure thorough mixing.
c) To calculate the volume of ammonium thiocyanate solution needed, assuming the maximum amount of Cu in the sample:
Step 1: Calculate the mass of Cu in the sample as calculated in part (a).
Mass of Cu = 0.1923 g
Step 2: Calculate the volume of ammonium thiocyanate solution needed.
Density = Mass / Volume
Volume = Mass / Density
Volume = 0.1923 g / 1.00 g/ml
Volume = 0.1923 ml
However, we need to add a 10% excess of the ammonium thiocyanate solution. So, multiply the volume by 1.1:
Volume with 10% excess = 0.1923 ml * 1.1
Volume with 10% excess = 0.2115 ml
Therefore, you would need to add approximately 0.2115 ml of the 10% ammonium thiocyanate solution with a 10% excess.
d) To calculate the % Cu in the sample:
Step 1: Calculate the mass of the sample.
Mass of the sample = 4.5167 g
Step 2: Calculate the % Cu in the sample.
% Cu in the sample = (Mass of Cu / Mass of the sample) * 100
% Cu in the sample = (0.1923 g / 4.5167 g) * 100
Therefore, the % Cu in the sample is approximately 4.26%.