An original sample of K-40 has a mass of 25.00 grams.
After 3.9 m 109 years, 3.125 grams of the original sample remains unchanged.
What is the half-life of K-40?
First step is to determine the remaining decimal amount.
3.125 grams /25.00 grams = 0.125
Second step is to determine the number of half lives.
(1/2)^n = 0.125
N log (1/2) = log 0.125
N = 3 years
Lithium 7 has an equal number of protons and neutrons.
The initial balanced equation of decomposition is:
2NaN3 —> 2Na + 3N2
Since 1 mole of a gas occupies 24dm^3 at room temperature and pressure:
72dm^3 / 24dm^3 = 3 moles of Nitrogen which required 2 moles of Sodium Azide.
Since moles = mass/Relative molecular mass or molar mass
2 moles x 65~ g/mol = 130g
For the empirical formula:
Element H N
% Mass 2.33% 97.7%
Mass/Mr 2.33 6.97
2.33 7~
Divide by lowest number
1 3
Therefore empirical formula is
H1N3
To convert grams to moles, we need the molar mass of the element or compound. remember you can determine the molar mass of a molecule by adding the atomic mass of each atom from the periodic table.
molar mass of Na= 23.0 g/ mol
130 g (1 mol Na/ 23.0 grams)= 5.65 moles or 6 moles.
