Question

which of the following contains the most atoms? you shouldnt need to do a calculation here.
10.0 g mg
10.0 g kr
10.0 g ne
10.0 g he
10.0 g ar

Explanation:

Step1: Recall the relationship between mass, molar mass and number of atoms

The number of atoms \(N = n\times N_A=\frac{m}{M}\times N_A\), where \(n\) is the amount of substance, \(m\) is the mass, \(M\) is the molar mass and \(N_A\) is Avogadro's constant. Given \(m = 10.0g\) (constant for all options), \(N\propto\frac{1}{M}\) (since \(N_A\) is constant). So, the element with the smallest molar mass will have the most atoms.

Step2: Compare the molar masses of the elements

The molar masses are: \(M(Mg)=24.31g/mol\), \(M(Kr) = 83.80g/mol\), \(M(Ne)=20.18g/mol\), \(M(He)=4.00g/mol\), \(M(Ar)=39.95g/mol\). Among these, \(He\) has the smallest molar mass.

Answer:

10.0 g He