3.117 CP - Chemistry: Molecular Nature Of Matter And Change

Question

In 1961, scientists agreed that the atomic mass unit (amu) would be defined as 1/12 the mass of an atom of ¹²C. Before then, it was defined as 1/16 the average mass of an atom of naturally occurring oxygen (a mixture of ¹⁶O, ¹⁷O, and ¹⁸O). The current atomic mass of oxygen is 15.9994 amu. (a) Did Avogadro’s number change after the definition of an amu changed and, if so, in what direction? (b) Did the definition of the mole change? (c) Did the mass of a mole of a substance change? (d) Before 1961, was Avogadro’s number 6.02×10²³ (when considered to three significant figures), as it is today?

Step-by-Step Solution

Answer

1. Avogadro’s number will become smaller.
2. Yes, it changed.
3. Yes, it changed.

No, it is different

Checking the change of Avogadro’s number

The mass in old scale would be,

$\begin{array}{l}{\mathrm{m}}_{\mathrm{old}}\left({}^{12}\mathrm{C}\right)=\frac{16\mathrm{amu}.12\mathrm{amu}}{15.9994\mathrm{amu}}\\ {\mathrm{m}}_{\mathrm{old}}\left({}^{12}\mathrm{C}\right)=12.00045\mathrm{amu}\end{array}$

In old system,

$\begin{array}{l}\frac{12.00045\mathrm{g}.6{.0221409.10}^{23}\mathrm{entities}}{12\mathrm{g}}=6{.0223668.10}^{23}\mathrm{entities}\\ \frac{6{.0223668.10}^{23}\mathrm{entities}}{6{.0221409.10}^{23}\mathrm{entities}}.100\mathrm{\%}=100.0037512\mathrm{\%}\end{array}$

Definition of mole change

There is a change in Avogrado’s number, the mole also changed. Yes, it is changed

Change in mass of mole of substance

Yes, it is changed due to the mole number.

Old Avogrado’s number

The old Avogadro’s number was 3×10^-4 bigger.