Inverse proportion

Question

4. At a constant temperature, the volume of a gas V is inversely proptional to its pressure p. By what percentage will the pressure of a gas change if its volume increases by 25%?

 

all I know is v = k /p 

pls help me find the constant and help me understand what to do next

Answer 1

Constant is pressure * volume, or pv.  If pressure increases volume must decrease, and vice versa.

 

New volume is 125% or 125/100 or 5/4 of old volume.  So new pressure is 100/125 (reciprocal of 125/100) or 4/5 (reciprocal of 5/4) or 80% of old pressure.  Pressure dropped by 1/5, or 20% (100-80).

Check:  1.25 * 0.80 = 1.00.