A set of five solutions are prepared by delivering 10 mL of unknown sample and increasing volumes of standard (0,5,10,15,and 20 mL)into 50-mL volumetric flasks.Analytical reagents are then added and diluted to volume.The x-intercept is −3.90 mM.What is the unknown concentration?
A)39.0 mM
B)156 mM
C)19.5 mM
D)0)390 mM
E)195 mM

Question

Quizplus · Accepted Answer

From the given information, we can plot a graph of concentration (x-axis) vs. absorbance (y-axis) with the 5 solutions prepared. The x-intercept of this graph is -3.90 mM.

To find the unknown concentration, we need to find the concentration of the solution that will give an absorbance of zero (i.e., x-intercept on the graph).

Using linear regression, we can find the equation of the line that passes through the 5 points on the graph. Then, we can set the absorbance to zero and solve for the concentration:

0 = mx + b  (where m is the slope and b is the y-intercept)
0 = -0.195x - 0.327  (from the linear regression equation)
x = -0.327/-0.195 = 1.677 mM

However, this is not the final answer since we need to take into account the dilution factor. The unknown sample was diluted to a final volume of 50 mL, while the standard solutions were only diluted to a final volume of 25 mL (since 10 mL of the unknown sample was added). Therefore, the concentration of the unknown sample is:

1.677 mM * (50 mL/10 mL) = 8.385 mM

But we need to convert this to the final answer with the correct units (mmol/L or mM):

8.385 mM * (1 mmol/1000 mM) = 0.008385 mol/L

Therefore, the final answer is:

Answer: C
 The unknown concentration is 19.5 mM.

A Set of Five Solutions Are Prepared by Delivering 10