Biochemistry help! So confused

Question

Question 1:

A 4-fold dilution of plasma is assayed spectrophotometrically for cholesterol.

Given the following spectrophotometic data, what is the concentration of cholesterol in the diluted plasma in micromol/mL? (DO NOT INCLUDE THE UNITS IN YOUR ANSWER)


A₅₀₀ standard = 0.77
A₅₀₀ sample = 0.28
C (standard) = 3.9mM

I went (0.28 / 0.77) x 3.9 = 1.418.. and then times this by 4 and got 5.672 but thats wrong

 

Question 2

1 ml of whole blood is collected and centrifuged to obtain a plasma sample. The volume of the plasma sample is 500 microlitres. A 1 in 3 dilution of the plasma is made and analysed using a cholesterol assay like the one you performed in the laboratory. 10 microlitres of both the diluted plasma sample and a cholesterol standard are assayed using 1mL of cholesterol reagent. Following incubation at 37 degrees Celcius, the samples are measured spectrophotometrically at 500nm.

If the absorbance of the plasma sample assay was 0.3, the absorbance of the cholesterol standard assay was 0.5, and the concentration of cholesterol in the standard was 2.3mM, what was the concentration of the cholesterol in the whole blood (in mM)?

(assume all of the cholesterol in the whole blood sample was recovered in the plasma; do not include units in your answer; give your answer to 2 decimal places)

 

Question 3

1 ml of whole blood is collected and centrifuged to obtain a plasma sample. The volume of the plasma sample is 400 microlitres. A 1 in 2 dilution of the plasma is made and analysed using a cholesterol assay like the one you performed in the laboratory. 10 microlitres of both the diluted plasma sample and a cholesterol standard are assayed using 1mL of cholesterol reagent. Following incubation at 37 degrees Celcius, the samples are measured spectrophotometrically at 500nm.

If the absorbance of the plasma sample assay was 0.9, the absorbance of the cholesterol standard assay was 0.6, and the concentration of cholesterol in the original whole blood sample was 3.0mM, what was the concentration of the cholesterol standard (in mM)?

(assume all of the cholesterol in the whole blood sample was recovered in the plasma; do not include units in your answer; give your answer to 2 decimal places)

 

Question 4

 

A 3-fold dilution of plasma is assayed spectrophotometrically for cholesterol.

Given the following spectrophotometic data, what is the concentration of cholesterol in the undiluted plasma in millimol/mL? (DO NOT INCLUDE THE UNITS IN YOUR ANSWER)

A₅₀₀ standard = 0.45
A₅₀₀ sample = 0.2
C (standard) =3.4mM

 

I got 4.53 for this and that was wrong.

 

Thank you so much if you can help me with any of these questions