Question

Copper(II) forms a 1:1 complex with the organic complexing agent R in acidic medium. The formation of the complex can be monitored by spectrophotometry at 480 nm. Use the following data collected under psuedo-first-order conditions to construct a calibration curve of rate versus concentration of R. Find the concentration of copper(II) in an unkown whose rate under the same conditions was 6.2x10^-3 A s-1. Also find the standard deviation of the concentration

Cu2+, ppm	Rate, A s-1
3	3.60E-03
5	5.40E-03
7	7.90E-03
9	1.03E-02

Answer 1

[Cu2+]	Rate,A
(in ppm)	s-1
3	0.0036
5	0.0054
7	0.0079
9	0.0103

Equation of the graph is : y = 0.0011 x + (2X10^-5)

To find the concentration of copper(II) in an unkown whose rate under the same conditions was 6.2x10^-3 A s^-1

so, y = 6.2x10^-3 A s^-1 and we have to find x

: y = 0.0011 x + (2X10^-5)   6.2x10^-3 = 0.0011 x + (2X10^-5) 0.0011 x = 6.18X10^-3   x = 5.6 ppm

So, concentration of copper(II) in an unkown  is 5.6 ppm

To find the standard deviation of the concentration

x	x - xmean	(x - xmean)2
3	-3	9
5	-1	1
7	1	1
9	3	9
x = 24		(x - xmean)2 = 20

x_mean = x / n = 24/4 = 6

Standard deviation, s =

s=    = 2.58

the standard deviation of the concentration = 2.58 ppm