Standard Addition Chemistry problem

[xsc614]

Homework Statement

Homework Equations

Not completely sure, but I believe equation is:
Cx = bCs/mVx
cx = concentration of sample
b= y-intercept
cs= concentration of standard
m=slope
Vx=volume of sample

The Attempt at a Solution

I plotted the values on a graph with Absorbance on the Y-axis and [Pb] added on the X-axis, and put a trendline extending through the X axis. Eqn I came up with is y=.209x+2.211

My teacher was very vague with standard addition and through searching the web it said the best way to figure out the initial concentration is through spiking the samples. I am really not sure where to else to go with this problem since my equation above has several variables that aren't given in the problem statement (volume, concentration of standard).

 

[chemisttree]

Well? X in this case represents the added lead in mg/mL. What would the value of y represent when x=0?