Question

In: Chemistry

solution, mL dissolved rock 1.              10 2.               10 3.        &nb

solution, mL dissolved rock
1.              10
2.               10
3.              10
4.              10
5.               10
                 mL 0.100M sodium metasilicate added
0
1.00
2.00
4.00
20.00
total volume
50
50
50
50
50
Si Signal (mA)
0.235
0.500
0.768
1.250
5.202
A 10.000g sample of rock containing quartz as the sole source of silicon is dissolved and diluted to 1.000 +/- 0.010 liters and analyzed by the method of standard additions. What is the wt% of quartz in the rock?

Solutions

Expert Solution

Concentration of sodium metasilicate added = 0.150 M

Solution

Total volume (mL)

Volume of 0.150 M sodium metasilicate added (mL)

Concentration of added sodium metasilicate in the final solution (M) = (vol. added/total vol)*0.150 M

Intensity of signal (mA)

1

50

0.00

0.000

0.235

2

50

1.00

0.003

0.500

3

50

2.00

0.006

0.768

4

50

4.00

0.012

1.250

5

50

20.00

0.060

5.202

Plot a graph between the signal intensity and the concentration of added metasilicate in the final solution as shown below.

The plot gives the signal intensity vs concentration of added metasilicate. The plot is linear. The concentration of silicon in the unknown sample is found by extrapolating the graph to zero.

Put y = 0 and obtain

82.508x + 0.2544 = 0

===> x = -(0.2544/82.508) = -0.003083

Since concentration cannot be negative, ignore the negative sign and write the concentration of silicon in the unknown sample as 0.003083 M.

Quartz has the chemical formula SiO2 and molar mass is 60.08 g/mol.

We took 10 mL of the solution from the prepared 1.0 L stock solution. Therefore, concentration of silicon in the prepared stock solution is (0.003083 M)*(1000 mL/10 mL) [1 L = 1000 mL] = 0.03083 M.

Since we have 1 L stock solution, therefore,

moles of Si present = (concentration in mol/L)*(volume in L) = (0.03083 mol/L)*(1 L) = 0.03083 mol.

Since silicon is present as quartz only, the mass of quartz present = (0.03083 mol)*(60.08 g/1 mol) = 1.85226 g.

1.85226 g quartz is present in 10.00 g sample; therefore, percentage of quartz in the given sample = (1.85266 g/10.00 g)*100 = 18.5226 ≈ 18.52

Ans: Percentage of quartz in the given sample = 18.52.


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