Answer: 2.58 days
Explanation:
Expression for rate law for first order kinetics is given by:
where,
k = rate constant = ?
t = age of sample = 6 days
a = initial amount of the reactant = 1 g
a - x = amount left after decay process
= 0.2 g
a) to find the rate constant
b) for completion of half life:
Half life is the amount of time taken by a radioactive material to decay to half of its original value.
The half life is 2.58 days
Answer:
3.14 grams of ammonium thiocyanate must be used to react completely with 6.5 g barium hydroxide octahydrate.
Explanation:

The balance chemical equation is :

Mass of barium hydroxide octahydrate = 6.5 g
Moles of barium hydroxide octahydrate = 
According to reaction, 2 moles of ammonium thiocyanate reacts with1 mole of barium hydroxide octahydrate. The 0.020635 moles of barium hydroxide octahydrate will react with:

Mass of 0.04127 moles of ammonium thiocyanate;

3.14 grams of ammonium thiocyanate must be used to react completely with 6.5 g barium hydroxide octahydrate