The decomposition of n2o5(g) —> no2(g) + no3(g) proceeds as a first order reaction with a half life 30.0s at a certain temperature.
if the initial concentration [n2o5]0 = 0.400 m, what is the concentration after 120 seconds?
a)0.050
b)0.200
c)0.025
d)0.100

Explanation:

The half-life (30.0 s) is the time it takes for half of the N₂O₅ to react.  

After one half-life, half (50 %) of the original amount will remain.  

After a second half-life, half of that amount (25 %) will remain, and so on.  

We can construct a table as follows:  

1. for the first question, we must find the mass of the anhydrous salt, mgso₄. the molar mass for mgso₄·7h₂o is 246.47 g/mol, while that of mgso₄ is 120.37 g/mol.7.834 g mgso₄·7h₂o * 1 mol mgso₄·7h₂o/246.47 g * 1 mol mgso₄/1 mol mgso₄·7h₂o * 20.37 g mgso₄/mol = 0.647 g mgso₄

2. mass of water = mass of sample - mass of anhydrous salemass of water = 7.834 - 0.647 = 7.19 gpercent water in hydrate = 7.19/7.834 * 100 = 91.74%