How many mL of the stock solution would be needed to make the different concentr

ID: 1034109 • Letter: H

Question

How many mL of the stock solution would be needed to make the different concentrations? A. Analytical Curve Method A stock solution of 590.0 ± 0.1 mg Ca(NO3)2.4H20 made in a 1-L volumetric flask (What is the uncertainty in the volume of a 1-L volumetric flask?) with pure water has been prepared for you. What is the concentration (in ppm) of the calcium in this solution? What is the uncertainty in the concentration? Work this out in your notebook before the lab begins, and have me check your answer before you start. Think carefully about how many significant figures you should have.

Explanation / Answer

1) The uncertainty of a 1 L volumetric flask is 0.300 mL, i.e, the volume of a 1 L flask is 1000 0.300 mL.

We define 1 ppm = 1 mg/L.

We added 590.0 mg Ca(NO3)2.4H2O to a 1 L volumetric flask; therefore, the concentration of Ca2+ [Ca(NO3)2 ionizes completely in water to give Ca2+ and NO3- ions] in the solution is 590.0 mg/1 L = 590.0 mg/L = 590.0 ppm (1 ppm = 1 mg/L).

The uncertainty in the mass of Ca(NO3)2.4H2O is 0.1 mg; therefore, the relative uncertainty in the mass of Ca(NO3)2.4H2O = (0.1 mg)/(590.0 mg) = 0.00016949.

The uncertainty in the volume of Ca(NO3)2 is 0.300 mL; therefore, relative uncertainty in the volume of Ca(NO3)2.4H2O = (0.300 mL)/(1000 mL) = 0.0003.

Relative uncertainty in the concentration of Ca2+ = ?[(0.0001694)2 + (0.0003)2] = 0.0003446.

Uncertainty in the concentration of Ca2+ = (0.0003446)*(590.0 ppm) = 0.203314 ppm ? 0.2 ppm. Therefore, the concentration of Ca2+ is 590.0 0.2 ppm (ans).

2) The final volume of the solution prepared is 50 mL. Use the dilution equation.

C1*V1 = C2*V2

where C1 = 590.0 ppm; C2 = 1 ppm and V2 = 50 mL (take solution 1 as example).

(590.0 ppm)*V1 = (1 ppm)*(50 mL)

=====> V1 = (1 ppm)*(50 mL)/(590 ppm) = 0.0847 mL ? 0.085 mL.

Prepare the table as below.

Solution

Concentration of Ca2+ in the final solution (ppm)

Volume of stock Ca2+ solution taken (mL)

0.085

0.170

0.339

0.508

0.678

0.847