Question for a tutor: I tried calculating the amount of Ca in the unknown by solving for x using the y=mx+b, but it was

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Question for a tutor: I tried calculating the amount of Ca in
the unknown by solving for x using the y=mx+b, but it was
incorrect. I also need help finding out the wt % of the Ca in the
unknown.
(1) How do I solve for the amount of Ca given in the original
sample in mg?
(2) How do I find out the wt % of Ca in the unknown?
*If you could please share your math, how your excel table looks
and how your graph look with the information provided (red
text in the problem), that'd be great!
Thank you so much!
Info from lab manual:
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Question:

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Experimental Procedure: Preparation of Unknown Sample Bring your lab notebook, a watch glass, a 50 mL beaker and 100 mL beaker to the TA. Obtain the Ca standard, the pH/ionic strength diluent and an unknown supplement from the TA. Weigh the supplement, so that at the end of the lab you can calculate and report the weight % of calcium in the unknown. After weighting, place the unknown in a 250 mL beaker and add 30 mL of 1.0 M HCl. Bring this solution to a slow boil and continue the boil for at least 15 minutes after the material has dissolved. You can expect to see a small amount of granular material left since the many of the binders, colorants and other additives are insoluble in water and acid. Some of these materials may be floating on the surface as a scum. Vacuum filter the solution, remembering that you're interested in the filtrate (the liquid in the flask) not the precipitate. The vacuum flask should be clean (not dry) before you start, and take extra care to ensure that none of your solution is lost into the vacuum line. Be sure to rinse the beaker and filter paper with sufficient volumes of water so you don't loose any sample. Quantitatively transfer the filtrate into a 250 mL volumetric flask and dilute to the mark. Transfer 10.00 mL of this solution into a 500 mL volumetric flask, add 25 mL of the buffer/ionic strength solution provided, and dilute to the mark. This solution is the sample which you will measure with your standards. Preparation of the Standard Solutions Use the 0.100 M CaCl2 stock solution provided to prepare standard solutions containing 1.00 x 10-2 M, 1.00 x 10-3 M, 1.00 x 104 M, and 5.00 x 10-4 M CaCl2. Since ISEs measure activity not concentration, it's important to make sure that all of your solutions have the same ionic strength. The Cat? ISE, like many ISE's, is also sensitive to pH, especially at low pH. The pH and ionic strength of the solutions will be adjusted with a solution that contains 1.2 M acetic acid/1.0 M sodium acetate, which will control ionic strength and act as a pH buffer. The standards will be prepared by serial dilution. Serial dilution means that a series of standards are made, each from the previous one. Serial dilution is often used for standards that span several orders of magnitude, but should be used carefully. Since each standard incorporates all of the experimental errors from the previous solutions, each solution's concentration in the series becomes more imprecise. Plan in advance what glassware you intend to use and record details of dilution procedure (solutions, volumes, etc.) in your lab notebook. Check your dilution scheme with a TA or instructor before you actually start the experiment. To prepare the solutions, you will need to get ~15 mL of 0.100 M CaCl2 (obtain this standard solution from a TA) and -50 mL of the ionic strength/pH buffer from the reagent bench.
Each solution should have the same total volume of buffer to keep ionic strength and pH constant. (a) 1.00 x 102 M CaCl2 - Pipette 10.00 mL of 0.100 M CaCl2 stock solution into a 100-mL volumetric flask. Add 5.0 +0.1 mL of the buffer and dilute to the mark with deionized water. Remember throughout to always rinse a pipet several times with the solution you plan to pipet. (b) 1.00 x 10-3 M CaCl2 - In a 100-mL volumetric flask, prepare a 1:10 dilution of solution “a” above. Don't forget to add the correct volume of the buffer to yield the same total concentration (i.e., 4.5 mL of the buffer; why not 5 mL?) Oregon State University Version 20130828 Chemistry 324 Calcium Determination Using Potentiometry 8.5 Experiment #8 (c) 1.00 x 10-4 M CaCl2 - In a 100-mL volumetric flask, prepare a 1:10 dilution of solution “b” above. Don't forget to add the correct volume of the buffer. (d) 5.00 x 10-5 M CaCl2 - Pipette 5.00 mL of solution “b” above into a 100-mL volumetric flask, add 4.75 +0.05 mL of the buffer, and dilute to the mark with deionized water.
2. [3/5 Points) DETAILS PREVIOUS ANSWERS MY NOTES This is your experimental data for Experiment #8, Determination of Ca in a Supplement by lon Selective Electrode. YOU MUST READ THE EXPERIMENTAL WRITE-UP AND COMPLETE THE PRE-LAB PRIOR TO DOWNLOADING THIS DATA!! Your unknown number is: 1365 Your sample weighed 1.2020 grams You follow the procedure making up your unknown. You made your standards, and when you measured them on the ISE meter produced the following data: Standard A has a meter reading of -49.59 mv Standard B has a meter reading of -75.95 mv Standard Chas a meter reading of -104.82 mV Standard D has a meter reading of -114.19 mv The unknown has a meter reading of -88.82 mv Produce and submit a spreadsheet with a PROPERLY LABELLED calibration curve graph with a linear least square calculation of the best fit line. Calculate the following in CLEARLY LABELLED cells in your spreadsheet. The slope = 27.615 The intercept - 7.1537 The R2 value = 0.999 The total amount of Ca in the unknown you were given at the start of the experiment = The wt% of Ca in the unknown = % |x mg Submit Answer