calculate the number of particles in the following:
(a)69 g of Na atoms. (b)56 g of He (c) 0.1 mole of atom.

1) Molar mass of sodium atom  = 23 g mol^​-1
23 g of sodium contains = 6.022$\times$10²³ particles
69 g of sodium = $\frac{6.022\times {10}^{23}}{23}\times 69$ = 18.066$\times$10²³ particles

2) Molar mass of helium = 4 g mol^-1
4 g of He contains = 6.022$\times$10²³ particles
56 g of He contains = $\frac{6.022\times {10}^{23}}{4}\times 56$ = 84.308$\times$10²³ particles

3) 1 mole of atom contains = 6.022$\times$10²³ particles 
0.1 mole contains = 6.022$\times$10²³ $\times$0.1 = 6.022$\times$10²² particles