Question

1. The concentration of ammonia in commercially available cloudy solution used for
cleaning was conducted in a laboratory.
25.00mL of the cloudy ammonia solution was pipetted into a 250.00mL conical flask.
50.00mL of 0.100N HCl was immediately added to the conical flask which reacted
with the ammonia in solution.
NH3 (aq) + HCl (aq) NH4Cl (aq)
The excess HCl was then titrated with 0.050M Na2CO3.
21.50mL of Na2CO3 was required for the titration.
2HCl (aq) + Na2CO3 (aq) 2NaCl (aq) + CO2 (g) + H2O (aq)
Calculate the concentration of the ammonia in the cloudy solution, in Molarity.

2. There are 9 points discussed in Analytical Method Validation. Consider a titration between an acid and a base with the use of an indicator, state which 3 ( amongst the 9) are most important and your reasons.

Answer 1

Question #59450, Chemistry / General Chemistry

Question:

1. The concentration of ammonia in commercially available cloudy solution used for cleaning was conducted in a laboratory. 25.00mL of the cloudy ammonia solution was pipetted into a 250.00mL conical flask. 50.00mL of 0.100N HCl was immediately added to the conical flask which reacted with the ammonia in solution.

NH3 (aq) + HCl (aq) NH4Cl (aq) The excess HCl was then titrated with 0.050M Na2CO3. 21.50mL of Na2CO3 was required for the titration.

2HCl (aq) + Na2CO3 (aq) 2NaCl (aq) + CO2 (g) + H2O (aq)

Calculate the concentration of the ammonia in the cloudy solution, in Molarity.

2. There are 9 points discussed in Analytical Method Validation. Consider a titration between an acid and a base with the use of an indicator, state which 3 (amongst the 9) are most important and your reasons.

Answer:

1. Let's calculate the excess of chloric acid:

$$\begin{array}{l} n(HCl) = 2 \cdot n(Na_2CO_3) = 2c(Na_2CO_3) \cdot V(Na_2CO_3) \\ n(HCl) = 2 \cdot 0.05M \cdot 21.5mL = 2.15\ mmol \end{array}$$

Then we can calculate the number of the moles of HCl that reacted with ammonia:

$$\begin{array}{l} n(HCl)' = n(HCl)_{tot} - n(HCl) = c(HCl) \cdot V(HCl) - n(HCl) \\ n(HCl)' = 0.1N \cdot 50.00mL - 2.15mmol = 2.85\ mmol \end{array}$$

As far as we see from the equation, the number of the moles of chloric acid is equal to the number of the moles of ammonia reacted:

$$n(HCl)' = n(NH_3) = 2.85\ mmol$$

Then, the concentration of ammonia in solution is:

$$c(NH_3) = \frac{n(NH_3)}{V(NH_3)} = \frac{2.85\ mmol}{25\ mL} = 0.1140\ \frac{mol}{L} = 0.1140\ M$$

2. The objective of the analytical procedure should be clearly understood since this will govern the validation characteristics which need to be evaluated. Typical validation characteristics which should be considered are: 1) Accuracy 2) Precision 3) Repeatability 4) Intermediate 5) Precision 6) Specificity 7) Detection Limit 8) Quantitation Limit 9) Linearity 10) Range. The most important validation characteristics for the titration with indicator method are: linearity, accuracy and precision. The accuracy of an analytical procedure expresses the closeness of agreement between the value which is accepted either as a conventional true value or an accepted reference value and the value found. The precision of an analytical procedure expresses the closeness of agreement (degree of scatter) between a series of measurements obtained from multiple sampling of the same homogeneous sample under the prescribed conditions. The linearity of an analytical procedure is its ability (within a given range) to obtain test results which are directly proportional to the concentration (amount) of analyte in the sample.

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