# Calculating Molar Concentration for Beers Law Plot

• jpd5184
In summary, the individual is trying to create a Beer's law plot and is struggling with finding the molar concentration of their diluted solutions. They have measured the absorbance of each diluted solution and have a standard solution with a known molarity. They are unsure of how to calculate the molar concentration and have attempted to do so by multiplying the molarity of the standard solution by the moles of the solution used and then dividing by the volume. However, they have been advised that they should divide the number of moles by the volume to get the molarity.
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?

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

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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.

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

jpd5184 said:
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.

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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

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

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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

jpd5184 said:
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.

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?

## What is the formula for calculating molar concentration in Beer's Law plot?

The formula for calculating molar concentration in Beer's Law plot is C = A/εl, where C is the molar concentration, A is the absorbance, ε is the molar absorptivity, and l is the path length of the cell.

## How do I calculate the absorbance for Beer's Law plot?

The absorbance for Beer's Law plot can be calculated using the equation A = log(I0/I), where I0 is the intensity of the incident light and I is the intensity of the transmitted light.

## What is the significance of the path length in Beer's Law plot?

The path length in Beer's Law plot refers to the distance that light travels through the sample. It is an important factor because it affects the amount of light that is absorbed by the sample, which in turn affects the absorbance and the calculated molar concentration.

## How do I determine the molar absorptivity for Beer's Law plot?

The molar absorptivity (ε) is a constant that is specific to each substance and is determined experimentally. It can be calculated by plotting a graph of absorbance versus concentration and finding the slope of the line, which is equal to ε.

## Why is Beer's Law plot important in scientific research?

Beer's Law plot is important in scientific research because it allows for the quantification of the concentration of a substance in a solution. This is useful in various fields such as chemistry, biochemistry, and environmental science, where accurate measurements of substances are necessary for experiments and analysis.

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