Nathaniel B. answered • 11/05/19
Bachelor of Science in Biology with a Minor in Chemistry
You have already listed the two steps you will need to find the answer.
a) Calculate the number of moles of yellow dye in 6.50 mL of the stock solution
We know that we are starting with 7.3 x 10-5 M stock solution, and we have 6.50 mL of this solution. Remember that Molarity (M) is expressed in units of mol/L. Looking at this, we should notice that multiplying the volume of the solution by the Molarity of our solution will give us the number of moles in our solution. Just make sure the units are all the same. In this example, we are given units in mL, so make sure to convert that over to Liters before attempting the first step:
7.3 x 10-5 M x 0.0065 L = 0.0000004745 or 4.745 x 10-7 moles of yellow dye
b) Calculate the concentration of the diluted solution.
As we have already discussed, Molarity is expressed in units of mol/L. We have mol, so we simply divide by the number of liters we have in our finished solution of 25.00 mL. Again, remember to remain consistent with your unit usage. It's mol/L not mol/mL, so make sure to turn that 25 mL into 0.025 L before attempting this step:
4.745 x 10-7 mol ÷ 0.025 L = 1.9 x 10-5 mol/L
Answer: The concentration of the dilute solution of yellow dye is 1.9 x 10-5 mol/L.
(Side Note - The ability to convert and cancel units is what this homework problem is truly trying to teach you. It is a great idea to practice conversions such as inches to feet to miles to kilometers, etc. as a way to get used to seeing these written out. Almost all of these problems can be solved with little to no knowledge of chemistry if you have a solid understanding of conversions. This practice will make problems like this so much easier in the long run.)
