Question

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Answer 1

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Formulas -

Number of moles
= $\frac{given \: mass }{molecular \: mass}$

Number of particles = moles ×$N_A$

Q1

(A) 62g of Ca = $\frac{62}{Atomic \: mass \: of \: Ca} moles$

=$\frac{62}{40} = 1.55 \: moles$

Number of particles = Moles × $N_A$

= 1.55 × 6.022 ×${10}^{23}$
= 9.33 × ${10}^{23}$ particles.

(B) Oxygen molecule =$O_2$

12 g $O_2$
=$\frac{12}{32}$ = 0.375 moles

Number of particles = Moles × $N_A$

= 0.375 × 6.022 ×${10}^{23}$
= 2.25 × ${10}^{23}$ particles.

2.

Concentration of a solution =
$\frac{Mass \: of \: solute}{Mass \: of \: solution}\times 100$

Mass of Solvent = 78
Mass of Solution = 528g
= Mass of solvent + Mass Of solute.
= 78 + 450
= 528g

Concentration =$\frac{78}{528} \times 100 = 14.77$

3.

Mole of aluminium oxide $(Al_2O_3)$
= 2 x 27 + 3 x 16
= 102 g

i.e., 102 g of $(Al_2O_3)$= 6.022 x ${10}^{23}$ molecules of $Al_2O_3$

Then, 0.083 g of $(Al_2O_3)$ contains = 6.022 x${10}^{23}$ / 102 x 0.083 molecules.

= 0.711 ×${10}^{23}$ molecules of $(Al_2O_3)$

The number of aluminium ions${Al}^{3 + }$present in one molecule of aluminium oxide is 8.4 .

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