Question

Solutions with a constant volume of 10.0 mL contain 0.750mg of dissolved tooth enamel plus variable concentrations of added Sr.

Added Sr(ng/mL=ppb) Signal(arbitrary units)
Added Sr: 0 , 2.50, 5.00, 7.50, 10.00

Signal: 28.0, 34.3, 42.8, 51.5, 58.6

a.) Find concentration of Sr and its uncertainty in the 10.0mL solution in ppb.
b.) Find concentration of Sr in tooth enamel in ppm.
c.) If the standard addition intercept is the major source of uncertainty, find the uncertainty in the concentration of Sr in tooth enamel in ppm.
d.) Find the 95% confidence interval for Sr in tooth enamel

I figured out a: 8.72 +/- 0.43 ppb, I just don't understand the rest

Answer 1

The graph with the given data is :

a) Solve the equation from the graph of the least – squares line for y= 0 to get x – intercept.

            X = - 27.36/ 3.136 = - 8.72 ng/mL

            Concentration of Sr⁺² = 8.72 ng/mL = 8.72 ppb.

b) In 10.0 mL of unknown solution there are (10.0 mL)*(8.72 ng/mL) = 87.2 ng of Sr⁺². Since the solution was made from 0.750 mg of tooth enmel, the concentration of strontium in the enamel = (87.2x10 ^{– 9} g of Sr⁺²/ 0.750x10 ^{– 3} g enamel)x (10⁶ µg/g)

c) Relative uncertainity of intercept is 100 x 0.43/8.72 = 4.9% , which leads to a 4.9% uncertainity in the concentration of Sr in the tooth enamel. 0.049 x 116ppm = 5.7 ppm. Final answer is 116 6 ppm.

3) 95% confidance interval is 3.182. The standard deviation = 5.7 ppm. 95 % confidance interval is =(3.182)x(5.7ppm) = 18.1 ppm. Answer is 116 18 ppm.