Substance a undergoes a first order reaction a  b with a half- life of 20 min at 25 °c. if the initial concentration of a ([a]◦ ) in a sample is 2.7 m, what will be the concentration of a after 60 min?

The concentration of A will be 0.34 mol·L⁻¹ after 60 min.

In a first-order reaction, the formula for the amount remaining after n half-lives is

If

∴

n+h→nh3

we must remember that nitrogen and hydrogen are both diatomic molecules in their standard gas form. this adjusts the equation to

n2+h2→nh3

now we need to adjust coefficients in order to balance the atoms on each side of the equation. currently we have 2 atoms of nitrogen and 2 atoms of hydrogen on the reactant side and 1 atom of nitrogen and 3 atoms of hydrogen on the product side.

we can balance the hydrogens by placing a coefficient of 2 in front of ammonia and a coefficient of 3 in front of the hydrogen.

n2+3h2→2nh3

this gives us 6 hydrogen on each side and coincidentally the nitrogens now equal 2 on each side.