A sample of gallium contains two isotopes only.
In every \(10\) atoms in the sample, there are \(6\) that have \(38\) neutrons and \(4\) that have \(40\) neutrons.
What is the relative atomic mass, \(A_{\mathrm{r}}\), of the gallium in the sample?
Correct Answer: \( \boxed{\mathrm{B}} \)
Gallium has atomic number \(31\).
Therefore, the two isotopes have mass numbers:
The relative atomic mass is:
\(\mathrm{\dfrac{(6\times69)+(4\times71)}{10}=69.8}\)
Therefore, the correct answer is (B).
Helium forms an ion \(\,^{3}_{2}\mathrm{He}^{+}\).
Three statements about this ion are listed.
1 It contains two protons.
2 It contains three neutrons.
3 It contains one electron.
Which statements are correct?
Correct Answer: \( \boxed{\mathrm{B}} \)
For \(\,^{3}_{2}\mathrm{He}^{+}\):
Therefore, statements 1 and 3 are correct, so the correct answer is (B).
A sample of magnesium contains the isotopes \(\mathrm{^{24}Mg}\), \(\mathrm{^{25}Mg}\) and \(\mathrm{^{26}Mg}\) only.
The percentage abundance of \(\mathrm{^{25}Mg}\) and \(\mathrm{^{26}Mg}\) is the same.
The relative atomic mass of magnesium in the sample is \(24.3\).
What is the percentage abundance of \(\mathrm{^{24}Mg}\)?
Correct Answer: \( \boxed{\mathrm{D}} \)
Let the percentage abundance of \(\mathrm{^{25}Mg}\) and \(\mathrm{^{26}Mg}\) each be \(x\%\).
Then the percentage abundance of \(\mathrm{^{24}Mg}\) is \((100-2x)\%\).
Using the weighted average:
\(\dfrac{24(100-2x)+25x+26x}{100}=24.3\)
Simplifying:
\(\dfrac{2400+3x}{100}=24.3\)
\(2400+3x=2430\)
\(3x=30\)
\(x=10\)
Therefore, \(\mathrm{^{24}Mg}\) has an abundance of \(100-2(10)=80\%\).
Therefore, the correct answer is (D).
