[tellyas]

Hello, I am very much struggling with the following:
Hello, I am desperate for the answers/help with this homework Please Help:

1) A calibration curve for CuSO4 at 635 nm yields a straight line described by the equation y = 3.107 x + 1.831. If the concentration of a sample of CuSO4 is 0.376 M, what is the predicted value of its absorbance? Provide your response to three digits after the decimal.

2) A calibration curve for CuSO4 at 635 nm yields a straight line described by the equation y = 4.548 x + 0.98. If the measured absorbance for a solution of unknown concentration of CuSO4 is 0.189, what is the concentration of the CuSO4 solution? Provide your response to three digits after the decimal. _____ M

3) A 0.133 M aqueous solution of CuSO4 exhibits an absorbance of 0.307. If the cuvette has a width of 1.589 cm, what is the molar absorptivity of the solute? Provide your response to three digits after the decimal. _______ L mol-1 cm-1

4) Distilled water (2.194 mL) is added to a cuvette for calibration of the SpectroVis. 3 aliquots, each 223.012 µL in volume, of 0.40 M CuSO4 are then added sequentially to the water in the cuvette. What is [CuSO4] after the third aliquot is added? Provide your response to three digits after the decimal.

5) The reported molar absorptivity of CuSO4 is 0.76 L mol-1 cm-1. If a solution of CuSO4displays an absorbance of 0.054 in a cuvette that is 0.615 cm in length, what is the concentration of the CuSO4 solution? Report your response to three digits after the decimal. _____ M

[Arkcon]

Greetings, tellyas:, I'd like to welcome you to the Chemical Forums, I can see you're new here, but I want to ask you to review our Forum Rules{click}.  You already accepted the rules when you signed up for our forum, and you have to follow them, whether you agree with them or not, or even if you're unaware of them.

We want to see you do some work, on your own, and we'll help.  We don't dump complete answers, not for anyone, not for a high school student, nor a hard-working professional, nor even the home chemist working on their newest invention.

Your questions have a common thread, we're glad to help you work with them.  You first two ask you to show how you can use a calibration curve. Picture a straight line on a graph, starting at zero, going through points 1,1 then, 2,2 and so forth.  You can easily predict what happens at 3, and 4. 

OK. So you're given lines that don't go through zero, that have a more complex relationship between variables.  But you can try to work with them.

[tellyas]

Thanks! I will definitely review the rules again. I understand what you are saying by picturing it on the graph, however, Im not sure the process I would even begin doing to try and solve these problems. We have not learned any of this yet!

[csmcfetridge]

Hi, I have the same questions with different numerical values, but I've gotten #3,4 &5 - i just need help with #1&2.

Question1)  A calibration curve for CuSO4 at 635 nm yields a straight line described by the equation y = 3.096 x + 1.556. If the concentration of a sample of CuSO4 is 0.776 M, what is the predicted value of its absorbance?

My work (incorrect apparently): slope(m)=A(absorbance)/C(concentration)
so A=m*C   -->   3.096*.776= 2.402=A   XXX   (the correct answer was 3.958)
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Question2)  A calibration curve for CuSO4 at 635 nm yields a straight line described by the equation y = 3.375 x + 0.879. If the measured absorbance for a solution of unknown concentration of CuSO4 is 1.344, what is the concentration of the CuSO4 solution?

My work: A/m=C    1.344/3.375= .398   XXX   (the correct answer was 0.138)

Like the first guy said, we haven't been taught this stuff yet and are not familiar with these graphs. Was my approach the right sort of idea?
If someone could tell me where I went wrong I'd appreciate it!

[Arkcon]

You appear to be trying to use a formula from your textbook, but your experimental data isn't written that way, and you're not converting correctly.

However, like the O.P., you've done a good job of convincing yourself that you don't know how to do this.  But its simple algebra, and your instructor probably justified in expecting you to be able to handle it.  Try this:  x and y, in the formula, y-mx+b, are what?