Answer:
6.27 [gr].
Explanation:
the required mass can be calculated as:
m[Ca(OH)₂]=M[Ca(OH)₂]*m[CaCl₂]/M[CaCl₂], where M - molar mass [gr/mol], m - mass [gr];
m[Ca(OH)₂]=9.4*74/111≈6.2(6) [gr].
Answer: 20.7 kJ
Explanation:
According to first law of thermodynamics:
=Change in internal energy
q = heat absorbed or released
w = work done or by the system
w = work done on the system=
{Work is done on the system is positive as the final volume is lesser than initial volume}
w = 4.51 kcal =
(1kcal = 4.184kJ)
q = +1.79 kJ {Heat absorbed by the system is positive}

Thus
for a system that receives 1.79 kJ of heat from surroundings and has 4.51 kcal of work done on it at the same time is 20.7 kJ
Answer: half life
Explanation: Radioactive decay follows first order kinetics and the time required for the decay of a radioactive material is calculated as follows:

t= time required
k= disintegration constant
x= amount of substance left after time t
a= initial amount of substance
when one half of the sample is decayed, one half of the sample remains and t can be represented as 
at
, 

