The region is located on an active oceanic plate
The given question is incomplete. The complete question is:What is the relative atomic mass of a hypothetical element that consists isotopes in the indicated natural abundances.
Isotope mass amu Relative abundance
1 77.9 14.4
2 81.9 14.3
3 85.9 71.3
Express your answer to three significant figures and include the appropriate units.
Answer: 84.2 amu
Explanation:
Mass of isotope 1 = 77.9
% abundance of isotope 1 = 14.4% = 
Mass of isotope 2 = 81.9
% abundance of isotope 2 = 14.3% = 
Mass of isotope 3 = 85.9
% abundance of isotope 2 = 71.3% = 
Formula used for average atomic mass of an element :

![A=\sum[(77.9\times 0.144)+(81.9\times 0.143)+(85.9\times 0.713)]](https://tex.z-dn.net/?f=A%3D%5Csum%5B%2877.9%5Ctimes%200.144%29%2B%2881.9%5Ctimes%200.143%29%2B%2885.9%5Ctimes%200.713%29%5D)

Therefore, the average atomic mass of a hypothetical element that consists isotopes in the indicated natural abundances is 84.2 amu
The quantity of NaOH required to reach the third equivalence point is 20mL.
Using the titration formula,
CaVa/CbVb = Na/Nb
Where,
Ca = concentration of citric acid (0.200 M)
Cb = concentration of NaOH (0.750 M)
Va = Volume of citric acid (25.0 mL)
Vb = volume of NaOH (x mL)
Na = number of reacting mole of citric acid (3)
Nb = number of reacting mole of NaOH (1)
Therefore Vb ( x mL) =CaVaNb/CbNa
= 0.2× 25×3/0.75 ×1
= 15/0.75
Vb ( x mL) = 20 mL
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