Why are isotopes with short half-lives not useful for dating very old rocks? -because not enough of the parent isotope remains to measure accurately
-because not enough of the daughter product has formed to be detectable
-because neither the parent isotope nor the daughter product will be detectable
-because very old rocks would never have contained these isotopes

because not enough of the parent isotope remains to measure accurately. because not enough of the daughter product has formed to be detectable.

Explanation:

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1. for the first question, we must find the mass of the anhydrous salt, mgso₄. the molar mass for mgso₄·7h₂o is 246.47 g/mol, while that of mgso₄ is 120.37 g/mol.7.834 g mgso₄·7h₂o * 1 mol mgso₄·7h₂o/246.47 g * 1 mol mgso₄/1 mol mgso₄·7h₂o * 20.37 g mgso₄/mol = 0.647 g mgso₄

2. mass of water = mass of sample - mass of anhydrous salemass of water = 7.834 - 0.647 = 7.19 gpercent water in hydrate = 7.19/7.834 * 100 = 91.74%

answerambulatory;