Question 1: Find the algebraic answer and absolute uncertainty and express each
ID: 991719 • Letter: Q
Question
Question 1: Find the algebraic answer and absolute uncertainty and express each answer with the correct amount of significant figures. 2.4 (± 0.5) 4.7 (± 0.1) × 5.5 (± 0.4) = ____± ____. Question 2: Five solutions were prepared in 10 mL volumetric flasks by adding 5 mL of unknown to the given volume of stock solution and diluting to 10 mL with acetic acid. Using a standard addition experiment set up, students plot their data on fluorescence vs. standard concentration (ppm), and find this information from the line: y = 0.0179x 0.101 1) Calculate, to three sig figs, the concentration of unknown in the solutions prepared. 2)Calculate to three sig figs, the undiluted concentration of unknown. Question 3: The following is a description of the preparation of an unknown sample: Solution 1. 0.0519g of an unknown Fe2 salt is measured and added to a 25 mL volumetric flask. 1.0 mL of 6 M HCl is added before diluting to volume. Solution 2. 1.0 mL of Solution 1 is then transferred to a 100 mL volumetric flask. 1.25 mL of buffer and 5.0 mL of ferrozine is added before diluting to volume. The absorbance of Solution 2 is measured to be 0.895. Report all answers to 3 sig figs. A) Calculate the concentration of Solution 2 (in mM) B) Calculate the concentration of Solution 1 (in mM). C) Calculate the molar mass of the unknown (prepared by adding 0.0519g to 25 mL). D) Assuming the cell path length is 1 cm, what is the molar absorptivity coefficient based on the calibration solutions prepared (in units of mM-1 cm-1)?
Explanation / Answer
RULE I: When two numbers are added, the uncertainty of each must first be expressed in absolute form. Then, the absolute uncertainty of the sum is the sum of the absolute uncertainties in the two numbers.
(A ± A) + (B ± B) = (A + B) ± ( A + B)
absolute uncertainties------------------^------^
RULE II: When two numbers are multiplied, the uncertainty of each must first be expressed in relative form. Then, the relative uncertainty in the product is the sum of the relative uncertainties in the two numbers:
[A ± A/A] x [B ± B/B] = (A x B) ± [ A/A + B/B]
relative uncertainties--------------------------^-------^
Now using the above two rules:
(Considering addition between the first two observations as there is no sign)
2.4 (± 0.5) + 4.7 (± 0.1) × 5.5 (± 0.4) = 2.4 (± 0.5) + (4.7 x 5.5) ± (0.1/4.7 + 0.4/5.5)
= 2.4 (± 0.5) + (25.85) ± (0.094)
= 2.4 (± 0.5) + (25.85) ± (0.094 x 25.85)
(Converting relative uncertainty to absolute uncertainty)
= (2.4 + 25.85) ± (0.5 + 2.429)
= (28.25) ± (2.929)
= 28.25 ± (2.9)
Thus the absolute uncertainty is 2.9 rounded to the significant figure.
Related Questions
drjack9650@gmail.com
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.