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 -> $\pu{2.5 g}$

  • $\begingroup$ The atomic mass unit (unit symbol: amu) is obsolete since 1961.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). $\endgroup$
    – user7951
    Jun 19, 2015 at 9:57

2 Answers 2


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


A gram-mole is the numeric value of atomic weight of the formula unit times grams. Given an amount of said stuff in grams, divide by the numeric value of atomic weight of the formula unit to output the number of gram-moles of stuff. One gram-mole contains Avogadro's number of formula units. Embroidering the answer atom-by-atom undoes the installed shortcut to arrive at the same answer the hard way. Avogadro's number is subject to small changes depending on what defines a kilogram.

A mole is the archetypal 5 pound bag. A mole of sulfur atoms is about 32 grams. Rhombic sulfur is eight-membered rings. A mole of those is (8)(32 grams). The crystallographic unit cell contains 16 rings. A mole of unit cells is (16)(8)(32 grams). If you are reacting zinc and sulfur, all you care about is moles of atoms. If you want to know how many unit cells you have for x-ray diffraction of a rhombic sulfur microcrystal, you want to take those atoms 128 at a time.


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