#### • Class 11 Physics Demo

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#### • decay constant of carbon

Question:(a) 209Pb decay of 209 Bi with a half life 3 1/4 hours. what is the rate for this decay /? (b) what is ment by the term half life and how would this be shown on a graph ?

Answers:From: Ln 2 = kt = 5730k, k = Ln 2/(5730y) = 0.0001210/year For 6.3 mg sample, the rate = -k(6.3mg) = -0.00076 mg/year. a) Rate can not be calculated without given amount of radioactive sample. b) Half life is one characteristic data showing how radioactive a material is. It means the time needed for a certain material to decay to half of its original amount. If you plot a graph of the amount of the material vs. time, the half life is the time at 50% of its original amount.

Question:Given a 11 pF air-filled capacitor, you are asked to convert it to a capacitor that can store up to 9.4 J with a maximum potential difference of 503 V. What must be the dielectric constant of the material that you should you use to fill the gap in the air capacitor if you do not allow for a margin of error?

Answers:Energy = 1/2*C*v^2 so to obtain this energy we need a capacitor of C = 2*energy/v^2 = 2*9.4x10^-6/503^2 = 7.43x10^-11F The capacitor is 11x10^-12F So the dielectric constant must be 7.43x10^-11/11x10^-12 = 6.76

Question:a 0.185M solution of a weak acid (HA) has a pH of 2.95. Calculate the acid ionization constant (Ka) for the acid.

Answers:[H+]= [A-]= 10^-2.95=0.00112 M Ka = [H+][A-]/ [HA]= 0.00112 x 0.00112 / 0.185 =6.81 x 10^-6

Question:The following is given. Calculate the value of the gas law constant, R. mass (mg, g) volume of gas (mL) barometric pressure (mm Hg) temperature (K) So I think the formula PV=nRT is used. My question is... isn't R always constant? (Equaling to 0.0821 L-atm/K-mol) So what am I supposed to solve/do?

Answers:yeah- pv=nrt p=pressure in pascals (pa) v= volume of gas- in dm3 n= number of moles r= gas constant t=temp in kelvin so R=(PV)/NT make sure you convert them into the correct units