
(a)
Interpretation:
For measurement giving y value of 2.58, the corresponding x value and its standard uncertainty has to be found out.
Concept Introduction:
Uncertainty with a Calibration Curve:
Propagation for the equation y=mx+b (but not y=mx) gives,
The standard uncertainty in x is given as:
ux=sy|m|√1k+1n+(y−ˉy)2m2∑(xi−ˉx)2
Where,
ux is standard uncertainty for x
y is the corrected absorbance of unknown
xi is the mass of sample in standards
ˉy is the average of y values
ˉx is the average of x values
k is the number of replicate measurements
(a)

Answer to Problem 4.33P
The corresponding x and its standard uncertainty value for y value of 2.58 is 2.0±0.3
Explanation of Solution
Given data:
Consider least-squares in Figure 4-11
A new single measurement gives a y value of 2.58
Finding of x value and its standard uncertainty:
The equation of straight line from figure 4-11 is y=mx+b⇒y=0.61538x+1.34615
On rearranging, we get,
x=y−bm =2.58−1.350.615 =2.00
The value of ˉy and ˉx is calculated as follows,
ˉy=(2+3+4+5)/4 =3.5ˉx=(1+3+4+6)/4 =3.5
The ∑(xi−ˉx)2 is found as follows,
∑(xi−ˉx)2=(1−3.52)+(3−3.52)+(4−3.52)+(6−3.52) =13.0
The standard uncertainty in x is given as,
ux=sy|m|√1k+1n+(y−ˉy)2m2∑(xi−ˉx)2 =0.19612|0.61538|√11+14+(2.58−3.5)2(0.61538)2(13.0) =0.38 =0.3(rounded off)
Therefore, the value for x is 2.0±0.3
The corresponding x and its standard uncertainty value for y value of 2.58 is found out as 2.0±0.3
(b)
Interpretation:
The four times measurement of y gives an average value of 2.58, the corresponding x value and its standard uncertainty based on four measurements has to be found out.
Concept Introduction:
Uncertainty with a Calibration Curve:
Propagation for the equation y=mx+b (but not y=mx) gives,
The standard uncertainty in x is given as:
ux=sy|m|√1k+1n+(y−ˉy)2m2∑(xi−ˉx)2
Where,
ux is standard uncertainty for x
y is the corrected absorbance of unknown
xi is the mass of sample in standards
ˉy is the average of y values
ˉx is the average of x values
k is the number of replicate measurements
(b)

Answer to Problem 4.33P
The corresponding x and its standard uncertainty value based on four measurements is 2.0±0.2
Explanation of Solution
Given data:
Consider least-squares in Figure 4-11
The y is measured four times and its average value is 2.58
The numbers in subscript denotes insignificant figures.
Finding of x value and its standard uncertainty:
The equation of straight line from figure 4-11 is y=mx+b⇒y=0.61538x+1.34615
On rearranging, we get,
x=y−bm =2.58−1.350.615 =2.00
The value of ˉy and ˉx is calculated as follows,
ˉy=(2+3+4+5)/4 =3.5ˉx=(1+3+4+6)/4 =3.5
The ∑(xi−ˉx)2 is found as follows,
∑(xi−ˉx)2=(1−3.52)+(3−3.52)+(4−3.52)+(6−3.52) =13.0
Here, the number of replicate measurement is four. Hence, k=4
The standard uncertainty in x is given as,
ux=sy|m|√1k+1n+(y−ˉy)2m2∑(xi−ˉx)2 =0.19612|0.61538|√14+14+(2.58−3.5)2(0.61538)2(13.0) =0.26 =0.2(rounded off)
Therefore, the value for x is 2.0±0.2
The corresponding x and its standard uncertainty value based on four measurements is found out as 2.0±0.2
(c)
Interpretation:
The 95% confidence intervals for (a) and (b) has to be found out.
Concept Introduction:
Confidence Intervals:
The confidence interval is given by the equation:
Confidence interval = ˉx±ts√n =ˉx±tux(since standard uncertainty(ux)=s/√n)
Where,
ˉx is mean
s is standard deviation
t is Student’s t
n is number of measurements
ux is standard uncertainty
To Find: The 95% confidence intervals for (a) and (b)
(c)

Answer to Problem 4.33P
The 95% confidence intervals for (a) is 2.0±0.8
The 95% confidence intervals for (b) is 2.0±0.6
Explanation of Solution
Given data:
The results of the measurement in part (a) is 2.00±0.38
The results of the measurement in part (b) is 2.00±0.26
The numbers in the subscript denotes insignificant figures.
Calculation of Confidence intervals:
The t value corresponding to 90% confidence level for three degrees of freedom is t90%=2.353
The 90% confidence interval for part (a) is calculated as follows,
confidence interval = ˉx±tux90% confidence interval =2.00(±2.353×0.38) =2.00(±0.89) =2.0(±0.8)(rounded off)
The 90% confidence interval for part (b) is calculated as follows,
confidence interval = ˉx±tux90% confidence interval =2.00(±2.353×0.26) =2.00(±0.61) =2.0(±0.6)(rounded off)
The 95% confidence intervals for (a) is calculated as 2.0±0.8
The 95% confidence intervals for (b) is calculated as 2.0±0.6
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Chapter 4 Solutions
Quantitative Chemical Analysis
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