[fredodgers]

Hi Everyone,

I have a quick question regarding a problem from class that I want to make sure I am doing correctly.  The actual question is the following:

The chromium in a series of steel samples was determined by ICP emission spectroscopy.  The spectrometer was calibrated with a series of standards containing 0, 2, 4, 6, and 8 μg K₂Cr₂O₇ per mL.  The instrument reading for these solutions were 3.1, 21.5, 40.9, 57.1, and 77.3, in arbitrary units. 

Additionally, the following data was obtained for replicate 1 g samples of cement dissolved in HCl and diluted to 100.0 mL.  The following are emission readings:
                         
                         Blank       Sample A             Sample B           Sample C

Replicate 1          5.1          28.6                        40.7                 73.1

Replicate 2          4.8          28.2                       41.2                 72.1

Replicate 3          4.9         28.9                         40.2                spilled

Calculate the percentage of Cr₂O₃ in each sample.



The way I did this was simply plotting the first 5 data points and getting a regression line.  After that, I subtracted the blank from each of the values in the above table and simply plugged in the average of each sample into the regression line equation I got (as the y-value) to get the concentration in μg/mL.  This was multiplied by 100 mL and this found mass was divided by 1g to get a percent.    Is that how this is done?  I was confused because it asked for percentage of Cr₂O₃, so I wanted to make sure of my process.  Thanks for the *delete me*

[Borek]

What you did is only partially correct. You should convert between masses of K₂Cr₂O₇ and Cr₂O₃.