# How many moles of the substance are in 2.5 g of lead?

How many moles of the substance are in 2.50 g of lead?

What I did:

Atomic mass of Pb: 207.2 amu (or g?)

Applying unitary method, 1 mol of Pb -> 207.2 g; so, x -> 2.5 g

$x = \frac{2.5}{207.2}= \pu{0.0121 mol}~\text{of}~\ce{Pb}$

Shouldn't it be like this?:

1 mol of Pb -> $(\pu{207.2 amu} \times1.66\times(10^{-24}))~\pu{g}$ so x -> $\pu{2.5 g}$

• The atomic mass unit (unit symbol: amu) is obsolete since 1961. The accepted units are dalton (unit symbol: Da) and the unified atomic mass unit (unit symbol: u). – Loong Jun 19 '15 at 9:57

One Pb atom weighs $\pu{207.2amu}$ or $\pu{207.2amu} \cdot \pu{1.66\times10^{-24}g}$.
But one $\text{mol}$ of Pb weighs $=\pu{207.2amu}\cdot \pu{1.66\times10^{-24}g}\cdot6.02\cdot10^{23}=\pu{207.2g}$