manganese-56 has a half life of 2.6 hours how much manganese would have disappeared after 7.8 hours if you started with 20g of manganese

17.5 g of manganese-56

Explanation:

Now we have to use the formula;

N/No = (1/2) ^t/t1/2

Where

N = mass of manganese-56 left after time (t) = ?

No = initial mass of manganese-56 present = 20 g

t = time taken for N mass of manganese-56 to remain = 7.8 hours

t1/2 = half-life of manganese-56 = 2.6 hours

Then we substitute values;

N/20 = (1/2) ^7.8/2.6

N/20 = (1/2) ^3

N = (1/2) ^3 * 20

N = 2.5 g of manganese-56

Mass of manganese-56 that has disappeared after 7.8 hours = 20 - 2.5 = 17.5 g of manganese-56