Ch4 Lec 8

Dilutions

Sec 4.5 : after Molarity...Dilutions of Solutions

When doing dilutions you need to keep in mind the number of moles that are being moved from one place to another. A typical dilution problem involves taking a certain volume of a solution of known molarity and then "diluting" this to a new volume. So if i have a solution of a molarity 1.23 M or 1.23 moles/L and i remove 0.0100 L of it, then i will have taken out a certain number of moles:

1.23 moles/L * 0.0100 L = 0.0123 moles

Now if these moles are diluted to a volume of 0.200 L, then the new concentration will be:

0.0123 moles/0.200 L = 0.0615 moles/L or 0.615 M

Since the number of moles removed from the original solution is the same as the number of moles in the final diluted solution, there is a shortcut to this calculation using:

M₁*V₁ = M₂*V₂

In the above calculation, M₁ = 1.23 M, V₁ = 0.0100 L, V₂ = 0.200 L and we are asked to determine M₂...so,

1.23 M * 0.0100 L = 0.200 L * M2 or M2 = (1.23 M* 0.0100 L)/0.200L ---> M2 = 0.0615 M

Dilution Worksheets

The following worksheets were originally planned as a pre-lab activity. I suggest that you look these Concentration, Molarity, and Dilutions worksheets over and to confirm your understanding, there is a WA; HW4.3 spring 2020 (Dilutions)...see WA for due date.

Dilution Activity

A lab was originally planned but is now being used as a class activity. In this activity/lab, you were going to determine the concentration of the blue dye found in a "blue" PowerAde sports drink.

Although the many details associated with the lab are outlined in the Molarity Lab, the basic take home message is...if you make a set of 3 solutions/dilutions of the blue dye and you use a spectrophotometer to measure how much light is absorbed by the solution (aka, the "absorbance") then you can generate a set of data to complete the following table:

If the [blue dye solution 1] = 0.010 M, [blue dye solution 2] = 0.020 M, [blue dye solution ] = 0.030 M (these are referred to as your "standards"), and solution 1 absorbance is 0.20, solution 2 absorbance is 0.40, solution 3 absorbance is 0.60, AND the sports drink absorbance is 0.51, then you can process this data as described below to determine the concentration of the blue dye in the sports drink.

Processing this Data

1) Write in your notes the above completed table.

2) Go to Excel (or by hand) and make a graph of concentration (x-axis) vs. absorbance (y-axis) for the standards...do not include the sports drink absorbance in this plot.

- fit a trendline to the data, showing the equation on the graph, note the equation in your notes.

Your trendline equation is in the form of y= m*x + b, or absorbance = m*concentration + b, where b should be 0, therefore, absorbance = m*concentration. The value for m (or the slope) is the conversion factor between the absorbance and concentration. NOW...plug in the absorbance for the sports drink into your trendline equation and then solve for the concentration:

Answer --> [blue dye]_{sports drink}= 0.025 M

3) Make sure your graph has axis labels, and a text box with your name and email me a screenshot of your graph...if you don't know how to take a screenshot (MAC command-shift-3 or command-shift-4, PC use app Snip-it).