[fobbz]

Consider the following reaction:
Zn_(s)+ 2HCl_(aq)--> ZnCl2_(aq)+ H2_(g)
Which of the following instruments could be used to monitor the rate of the reaction in an open system?

I.   Balance
II.  Pressure Gauge
III. pH meter

   A  I, II and III.
   B  I and II only.
   C  I and III only.
   D  II and III only.

I do not understand why the answer is not A. Since the reaction produces a gas, would it therefore not be ideal to measure the pressure of the system with a pressure gauge? More gas = more pressure right?

[enahs]

If the system is open to the atmosphere, you can not measure a pressure change. If you close the system to build up pressure, was does Le Chatelier principle say?

[fobbz]

If the system is open to the atmosphere, you can not measure a pressure change. If you close the system to build up pressure, was does Le Chatelier principle say?

Oh my, I am a total moron. I missed the most important part of the question when reading. As for your question I'm not sure of the answer, however the question is not in eq, so doesn't lechatlier's not apply?

[lillybeans]

Consider the following reaction:
Zn_(s)+ 2HCl_(aq)--> ZnCl2_(aq)+ H2_(g)
Which of the following instruments could be used to monitor the rate of the reaction in an open system?

I.   Balance
II.  Pressure Gauge
III. pH meter

   A  I, II and III.
   B  I and II only.
   C  I and III only.
   D  II and III only.

I do not understand why the answer is not A. Since the reaction produces a gas, would it therefore not be ideal to measure the pressure of the system with a pressure gauge? More gas = more pressure right?

Answer should be C, I and III only. You can't measure pressure in an open system (gas is escaping)! However, pH works because of HCl, which dissociates to form H+ and Cl-. As the reaction proceeds, H+ is consumed, so pH should go up. Balance works too because Zn(s) is slowly decomposing to form Zn(aq), also H2(g) is escaping so there should be an overall decrease in mass.