In our lab we generated and collected a sample of hydrogen gas over water by the

Question

In our lab we generated and collected a sample of hydrogen gas over water by the reaction of magnesium with HCl to find the gas constant R(ideal gas law). How would each of these affect my value?

10. How would the experimental (your!) value ofR be affected by the following conditions? State whether would be artificially (incorrectly) high, artificially low, or neither and give a brief explanation Example: There was a very small air bubble (about 0.1 mL) inside the inverted graduated cylinder before y started the reaction. Answer: R = (P * V)/(n * T Due to the bubble, V is larger than what it should be due to the formation of H2. Therefore, R appears to be artificially high. a) There was a large amount of air (5 mL) inside the inverted graduated cylinder before you started the reaction. b) A chunk of magnesium remained unreacted under the graduated cylinder c) The real temperature of the gas was higher than the measured temperature of the water over which the i collected. d The real air pressure inside the lab is lower than the measured atmospheric pressure you used in your calculation.

Explanation / Answer

According to the given data and ideal gas equation:

R = P*V/(n*T)

Note: The value or R is independent of the (i) nature of the gas, (ii) pressure (P), (iii) volume (V), (iv) no. of moles (n) and (v) the temperature (T).

It depends only on the units of expressing P and V.

If P is expressed atm, V is expressed in L, then R = 0.0821 L atm mol-1 K-1

Similarly, R = 8.314*107 erg mol-1 K-1 = 8.314 J mol-1 K-1, 1.987 ~ 2 cal mol-1 K-1.

Hence, for all the conditions mentioned in a), b), c) and d), the value of R is constant.

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