7. Define the following terms.
a) Stereoselectivity
b) stereospecific​

Explanation:

a) Stereoselectivity = The condition of being stereoselective or a measure of the extent to which a reaction is stereoselective is stereoselectivity.

b) stereospecific = The property of a chemical reaction that yields specific stereoisomers as products.

There are several information's already given in the question. Based on the information's the answer can be easily deduced. 

We know the formula

P1*V1/T1 = P2*V2/T2

As the temperature is constant, so T1 and T2 can be negated. The formula changes to 
P1*V1 = P2*V2
70 * 1 = 540 * V2
270/540 = V2
V2 = 0.5

From the above deduction, we can conclude that the new volume is 0.5 liters. I hope that the procedure is clear enough for you to understand.