Calculating Molar Concentration for Beers Law Plot

[jpd5184]

hello everyone, i need to do a beers law plot. i got the percent transmittance which i converted to absorbance but I am not sure how to get the molar concentration.

i found the max wavelength to be 515 nm. I prepared three 8-10ml diluted solutions by mixing a standard solution x (0.100M) and distilled water in ratios of 3:1(6ml standard solution, 2ml water),1:1(5ml of standard solution, 5ml water), and 1:3(2ml standard solution, 6ml water). i measured the absorbance of each of these diluted solutions and the max wavelength and got

3:1 diluted solution 0.294
1:1 diluted solution 0.466
1:3 diluted solution 0.143

i know that molarity = (moles of solute)(liters of solution)

how do i know what the liters of solution is and the moles of solute?

 

[Borek]

If you mix 5 mL with 2 mL, what volume will you get? (at least approximately)

 

[jpd5184]

it would be 7ml. so would i just multiply the added volumes by the molar concentration to get molarity.

i tried this but when i graphed it, it was not a straight line which it should be.

 

[Borek]

would i just multiply the added volumes by the molar concentration to get molarity.

You are multiplying concentration by volume to get concentration?

Sorry, but that's so off it is not even wrong.

http://www.chembuddy.com/?left=concentration&right=dilution-mixing

 

[jpd5184]

sorry,

its molarity=(moles of solute)(liters of solution)


the one thing i don't get is the beers law graph is absorbance vs molar concentration. so i know the liters of solution for each but don't know the moles.

moles of solute=(molarity)/(liters of solution)

would this be correct

 

[Borek]

sorry,

its molarity=(moles of solute)(liters of solution)

This is still wrong, you should divide, not multiply. Concentration is an amount of substance per volume.

 

[jpd5184]

i know its division i could have sworn i put the backslash in there but i quess not. so i know the equation so what volume do i use. would it be the 8ml,10ml, and 8ml in the three solutions

 

[Borek]

8/10/8 looks like a correct final volumes.

 

[jpd5184]

so i have the volume and absorbance of each solution, and the molarity of the standard solution.

to graph absorbance vs molarity i need molarity of each solution. to find this out i need to know the moles of each solution so i can divide it by liters to get molarity.

so would i just take the molarity of the standard solution and divide that by the volume of each diluted solution to get moles. but then what volume do i divide this by to get molarity.


just not sure if I am thinking correct or not

 

[Borek]

so would i just take the molarity of the standard solution and divide that by the volume of each diluted solution to get moles. but then what volume do i divide this by to get molarity.

No, you don't divide molarity by volume. You divide NUMBER OF MOLES by volume.

And number of moles is volume times concentration. C=n/V, simple algebra and solving for whichever variable is all you need.

 

[jpd5184]

so this is what i did.

i took the moles of standard solution used and multiplied by the molarity of the standard solution to get a number and then divided that number by 100ml to get my molarity

does that sound correct?