The continental crust is considered to be rich in iron and magnesium. What

conclusion can you reach about the amounts of these elements that are

necessary to enrich a substance? Explain.

To enrich a substance with these elements, only small amounts of these elements are necessary, because although the concentration of these substances is low they can be considered enriching.

Explanation:

The continental crust is formed by the continents and the shallow sections of the sea. The continental crust has a composition mainly formed by SiO2, Al2O3, Fe2O3, FeO, MgO, CaO, Na2O and K2, O. Regarding the question, all the substances that make up the continental crust are important and although iron and magnesium are in low concentrations, all these elements are enriching.

The answer is 4.

Half-life is the time required for the amount of a sample to half its value.
To calculate number of half-lives, we will use the following equation:

where:
n - a number of half-lives
x - a remained fraction of a sample

We do not know n and x.
n=?
But, we can calculate x (a remained fraction of a sample). If sample contains 10.5 g of the radioisotope Pb-212 and 157.5 g of its daughter isotope, Bi-212, there is in total 157.5 g + 10.5 g = 168 g of the sample. Of these 168 g, 10.5 g remained of the radioisotope Pb-212. The fraction is 10.5 ÷ 168 = 0.0625.
Thus, x = 0.0625







 


Therefore, the number of half-lives is 4.