For 1.9 mole of the radioactive isotope iron-59, half-life = 45.1 days?

The natural period will be 45.1/ln 2 days = 65 days = 93,694 minutes. Eleven minutes is small with respect to this, so it is sufficiently accurate to suppose that we have lost 11/93,694 of the original amount. Multiply this by 1.9 x 6.023E23 to get the number of disintegrations.