Concentration of Radon in atoms per cubic meter

[Ryomega]

Homework Statement

i) The average concentration of ²²²Rn in air inside a building is 20Bqm^-3; what is the average concentration of Rn in atoms per cubic meter?

ii) In a sealed room of volume 50m³ what activity of ²³⁸U in a piece of porous rock would eventually give rise to this ²²²Rn concentration? Assume that all of the ²²²Rn produced in the rock escapes into the air.

Homework Equations

R=λN
N=N₀exp[-λt]
τ=1/λ

The Attempt at a Solution

i) 20Bqm^-3 exists in air. Since R=λN

N=R/λ but: N = mass*N_A/molar mass

unknown mass, unknown decay constant...

Different approach

λ=ln(2)/half life

unknown half life, no given mean life. Stuck.

I'm sure I'm being stupid... I just don't see it.

ii) Transition rate problem...not sure where to start from. Any equations I'm missing?

Please help. Thanks in advance.

 

[phyzguy]

You need to look up the half-life of Rn-222 in order to do the problem.

 

[Ryomega]

It's an exam paper. All information I have is what I've written down. There must be a different approach. (unless it says in the data booklet but I doubt it)