Establish the atomic weight of the element by checking the periodic table. For example, the atomic weight of Cobalt-60 is 59.92 and Uranium-238 has the atomic weight 238.

Convert the mass into moles, using the formula moles = mass of the element / atomic mass of the element, and then convert moles to atoms by multiplying the mole value by Avogadro's number, 6.02 x 10^23. For example, to establish the number of atoms in 1 gram of Cobalt-60, calculate (1 / 59.92) x (6.02 x 10^23). This resolves to 1.01 x 10^22 atoms.

Substitute the activity of the element, for example 1.10 x 10^3 Ci for Cobalt-60, into the formula: r = activity rate x (3.700 x 10^10 atoms/s/Ci). The result is "r," the number of atoms decaying per second. For example, 1.10 x 10^3 x 3.700 x 10^10 = 4.04 x 10^13 atoms decaying per second, so r = 4.04 x 10^13.

Use the first-order rate equation, r = k[number of atoms]1, to determine the value for k. For example, using the values for "r" and the number of atoms previously determined for Cobalt-60, the equation becomes: 4.04 x 10^13 atoms decaying per second = k[1.01 x 10^22 atoms]. This resolves to k = 4.1 x 10^-9 s^-1

Determine the decay activity, in atoms / second, for the element. To do this, substitute the number of atoms in the sample into the equation: (4.1 x 10^-9 s^-1) x (number of atoms in the sample). For example, with 1.01 x 10^22 atoms the equation becomes : (4.1 x 10^-9 s^-1) x (1.01 x 10^22). This resolves to 4.141 x 10^13 atoms / second.

Calculate the value in curies by dividing the decay rate per second by 3.7 x 10^10, the decay rate equal to 1 curie. For example, 1 gram of Cobalt-60 is equivalent to 1,119 curies because 4.141 x 10^13/ 3.7 x 10^10 = 1,119 Ci.