Because 25% is half of 0.5 and the half life of is given as 5,715, the time it takes to reach this amount should just be double the half life: 11430 years.
However, in other cases the problem may not have easy numbers, so below is a more mathematical approach.
The half life formula is .
Solving for t gives:
Given information:
Plug in given values and solve for t.
With correct significant figures, this would be 11000 years.
Explanation:
(a)
This is acid base reaction because there is no change of oxidation state on either side of the reaction.
(b)
This is a oxidation reduction reaction because sodium in elemental state ( 0 oxidation state) oxidizes to Na⁺ in NaCl. Also H⁺ in HCl reduces to H° in H₂.
(c)
This is a oxidation reduction reaction because magnesium in elemental state ( 0 oxidation state) oxidizes to Mg²⁺ in MgCl₂. Also Cl° in Cl₂ reduces to Cl⁻ in MgCl₂.
(d)
This is acid base reaction because there is no change of oxidation state on either side of the reaction.
(e)
This is a oxidation reduction reaction because phosphorous in P³⁻ in K₃P oxidizes to P⁵⁺ in K₃PO₄ and oxygen reduces.
(f)
This is acid base reaction because there is no change of oxidation state on either side of the reaction.