A 100-ml volumetric flask was calibrated by repeatedly weighing the amount of wa

Question

A 100-ml volumetric flask was calibrated by repeatedly weighing the amount of water required to fill the flask exactly to its mark.

A. For the first trial, the initial mass of the clean dried volumetric flask was 78.8675 grams. The mass of the flask when filled to the mark was 177. 8311 g and the temperature of the water used was 18.0 degrees Celsius. Find the calibrated volume held by the volumetric flask.

B. After all the trial including the one in part (a). The average volume contained was found to be 100.104-ml with an uncertainty of +,- 0.009-ml. Is the calibrated volume accurate? Is it precise? As part of your answer, explain the objective criteria which you are using to make this determination.

C. Explain the process for calibrating a buret or other graduated instrument. As part of your answer, express how this is different from calibrating a normal volumetric pipet.

Explanation / Answer

A. Initial mass of flask = 78.8675 g

Mass of filled flask = 177.8311 g

Mass of water = Mass of filled flask - Initial mass of flask

= 177.8311 g - 78.8675 g = 98.9636 g

Density of water at 18.0 oC = 0.9986 g/ml

So, calibrated volume of water = mass/density = 98.9636/0.9986 = 99.102 ml

B. Average volume contained = 100.104 ml with an uncertainty of +,- 0.009-ml.

100.104 ml + 0.009 ml = 100.113 ml

100.104 ml - 0.009 ml = 100.095 ml

This means that the volume is between 100.095 ml to 100.113 ml.

The measurement is accurate when the volume measured is close to the actual measurement

So, we got volume between 100.095 ml to 100.113 ml and actual volume is 100.000 ml. So, we can say that the measurement is not accurate, as the first and second place after decimal are also different.

The measurements are called precise, if we have the values in all the trials same. But we are getting the uncertainity of 0.009 ml. This means that the measurements are precise, just the difference is in the last digit, which is acceptable.

So, the measurements are precise but inaccurate.

C. The process of calibration of burette:

1. Clean the burette properly with water and dry it. Fill it with water and make sure there are no bubbles in it.

2. Flow the water from the burette till the 0.00 ml mark on the burette. Clean the tip of burette with tissue and let the burette stand for 5 min. After 5 min, the reading should be same. If it is not same, it means that the burette is leaking and we need to tight the stopcock. Repeat the step again after tightening. If the volume remains same, then the burtte is functioning properly.

3. Take a weighing flask and clean it properly. Weigh the flask with the stopper.

4. Let 10.00 ml water flow from the burette to the flask. Weigh the flask and calculate the mass of water in flask. Keeping temperature in consideration, calculate volume of water with the help of density. The volume should be near to 10.00 ml.

5. Repeat the process 5 times to calculate precision and accuracy.

So, this was the process of calibration.

In case of pipet calibration, the pipet is dipped in the water and water is sucked in it. Then the water is allowed to flow till the mark and the water is then put in a clean dried preweighed flask. Rest of the procedure is same. We have to make sure, that the water is not sticking outside the pipet otherwise, the measurements will be wrong.

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