In an atom of an element there are a fixed number of electrons. If the total mass of electrons in 1 mole (6.022 x 1023) of atoms of a certain element is 18.65 mg, how many protons must there be in the nucleus of that atom? (mass of electron = 9.109 x 10–28 g).

No. of protons = 34

Explanation:

First we need to calculate the number of electrons in one mole of the the element:

No. of electrons per mole = Total Mass of Electrons/Mass of 1 Electron

No. of electrons per mole = (18.65 x 10⁻³ g)/(9.109 x 10⁻²⁸ g)

No. of electrons per mole = 2.04 x 10²⁵ electrons/mol

Now, we calculate the no. of electrons in 1 atom:

No. of electrons per atom = No. of Electrons per mole/No. of atoms per mole

No. of electrons per atom = (2.04 x 10²⁵ electrons/mol)/(6.022 x 10²³ atoms/mol)

No. of electrons per atom = 34 electrons/atom

Since, the no. of protons in a pure element are equal to the number of electrons. Therefore,

No. of protons = 34

there are two significant figures.