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 -> $\pu{2.5 g}$
Do not mix amu with mass of mol of substance! In your last formula you've forgot about Avagadro number.

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 $=\pu{207.2amu}\cdot \pu{1.66\times10^{-24}g}\cdot6.02\cdot10^{23}=\pu{207.2g}$