5. Avoid splashing water out of the cylinder, and shake gently to remove any air

Question

5. Avoid splashing water out of the cylinder, and shake gently to remove any air pockets. 6. Finish filling the cylinder to the 45-mL mark of the cylinder (like the (A)). Take a final buret reading. 7. Repeat the procedure for a total of three trials. PRE-LAB QUESTIONS (RESPOND ON A SEPARATE SHEET OF PAPER): The laboratory manual specifies two types of errors in measurements based on the cause of the error. What are these categories? Briefly describe each kind and provide examples. 1. 2. What is meant by the terms (a) accuracy and (b) precision? Define and provide examples. 3. Define the term absolute uncertainty. Give an example (not from the above reading). 4. Give a definition and an example of relative uncertainty (not from the above reading). 5. Define density and discuss what it means for this property to be an intrinsic property of a material. Look up (and record in your laboratory notebook) the literature values for the densities cf aluminum, lead, and steel. Include your reference. 6. Be sure to get the report from the course website and complete all sections.

Explanation / Answer

1) Experimental errors are common to every experiment. There are two types of experimental errors.

i) Random Errors are unpredictable changes that may occur in the course of an experiment. For example, a common random error occurs while reading the volume of a liquid with a graduated vessel. If the volume of the liquid lies between two graduation marks, the experimenter has to approximate the volume and this introduces a random error. A random error is one over which the experimenter has no control and affects the accuracy of an experiment.

ii) Systematic Errors are mostly instrumental errors caused by faulty instruments. For example if a weighing balance is not properly calibrated, then the balance will output erroneous results. Systematic errors are those which the experimenter can easily detect and can take measures to overcome.

2) (a) The accuracy of an experiment is the closeness of the experimentally obtained value of an experimental parameter to the true or accepted value of the parameter. For example, suppose, a student performs the ideal gas law experiment to determine the value of the gas constant, R. The accepted value of the gas constant is 8.314 J/mol.K. Supposed the student obtains a value of R as 8.287 J/mol.K; then the accuracy of the experiment is determined by comparing the true value to the obtained value.

(b) The precision of an experiment is the closeness of the individual value to each other. Suppose the student in part (a) performs three trials of the same experiment and obtain the following values of R.

R = 8.287, 8.301, 8.243 J/mol.K

The precision is simply the closeness of the values to each other and is a measure of the “repeatability” or “reproducibility” of the experiment.

3) The absolute uncertainty or absolute error of an experiment is the deviation of the experimentally obtained value of a parameter to the true or accepted value of the parameter. Suppose a student performs an experiment to determine the molar volume of a gas at STP and obtain a value of 22.9 L/mol. The accepted value of the molar volume of an ideal gas is 22.4 L/mol. The absolute uncertainty is obtained as

= (accepted value) – (obtained value) = (22.4 – 22.9) L/mol = -0.5 L/mol.

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