Question

Determine the number of moles in 4.75 X 1020 atoms of Lead

Answer 1

Answer:

$\boxed {\boxed {\sf 7.89*10^{-4} \ mol \ Pb}}$

Explanation:

To convert from moles to atoms, we must use Avogadro's Number. Avogadro's Number

• 6.022*10²³
• The number of particles (atoms, molcules, ions, etc.) in 1 mole.
• In this case, it is the number of atoms of lead.

1. Set up ratio

We can use Avogadro's Number as a fraction or ratio.

$\frac{6.022*10^{23} \ atoms \ Pb}{1 \ mol \ Pb}$

2. Convert atoms to moles

Multiply the given number of atoms by the ratio.

$4.75 *10^{20} \ atoms \ Pb *\frac{6.022*10^{23} \ atoms \ Pb}{1 \ mol \ Pb}$

Flip the fraction so that the atoms of lead can cancel each other out.

$4.75 *10^{20} \ atoms \ Pb *\frac{1 \ mol \ Pb}{6.022*10^{23} \ atoms \ Pb}$

$4.75 *10^{20} *\frac{1 \ mol \ Pb}{6.022*10^{23} }$

$\frac{4.75 *10^{20} \ mol \ Pb}{6.022*10^{23} }$

$7.88774494*10^{-4} \ mol \ Pb$

3. Round

The original measurement of 4.75 had 3 significant figures (4, 7, and 5).

We must round our answer to 3 sig figs, which is the hundredth place for the number found.

$7.88774494*10^{-4} \ mol \ Pb$

The 7 in the thousandth place tells us to round the 8 up to a 9 in the hundredth place.

$7.89*10^{-4} \ mol \ Pb$

There are 7.89*10⁻⁴ moles of lead in 4.75*10²⁰ atoms of lead.