[Cat Lady]

Question 1
A. In order to measure calcium in cereal by atomic spectroscopy 0,521 g of cereal sample was dissolved in small amount of HCl, quantitatively transferred to a volumetric flask, and diluted to 100 mL with water. Then 5 mL of this solution were transferred in each of eight 50 mL-volumetric flasks. Then different volumes of the standard solution containing 20 μg/mL of Ca2+ (see the table) were added into each of those eight flasks and diluted with water up to mark. The prepared solutions were analysed by atomic absorption spectroscopy and the recorded signal is given in the table below. Construct a standard addition graph and calculate %Ca (weight percent) in the cereal sample. 5 p

Flask              Ca2+ standard, mL         Measured absorbance

1                     0                                   0.151

2                     1                                   0.185

3                     3                                   0.247                   

4                     5                                   0.300

5                     8                                   0.388

6                    10                                  0.445

7                    15                                  0.572

8                    20                                  0.723


I constructed a graph with the concentration on the x axis and absorbance on the y axis,
is it true that the x intercept is the concentration of the unknown in the sample and which volume should I use to get the number of moles so that I can calculate the mass.

[Borek]

is it true that the x intercept is the concentration of the unknown in the sample

That's the idea behind the standard addition method.

and which volume should I use to get the number of moles so that I can calculate the mass.

You need to keep trace of dilution steps taken to calculate concentration in the original 100 mL, then use this concentration and 100 mL to calculate amount of calcium.

[Cat Lady]

y=kx + m

y(0)=0,151   m =0,151
y(4 microgram/mL)=0,445

0,445 = 4k + 0,151        k=0,0735
y=0=0,0735x + 0,151     x=-2,054 microgram/mL

This is the concentration without added standard which gives the total amount of Ca 50 * 2.054 microgram

This is five procent  out of the total sample.  Which provides that the total amount is  20*50*2,054
microgram= 2,054mg

 2,054/521=0,39%

Is that correct.

[mjc123]

Your calculation is correct; however, you do not use all the data that you were given. You will get a more reliable answer if you plot the graph using all 8 points and do a linear regression (easy in Excel). Use the regression values of the slope and intercept to calculate the concentration of the unknown. This is a more accurate method because (due to experimental variability) your two chosen points do not sit exactly on the regression line.
Doing this I get an answer of 0.43% - see if you agree!