There are two naturally occurring isotopes of gallium: mass of Ga-69 isotope is 68.9256 amu and its percentage abundance is 60.11%, let the mass of other isotope that is Ga-71 be X, the percentage abundance can be calculated as:
%Ga-71=100-60.11=39.89%
Atomic mass of an element is calculated by taking sum of atomic masses of its isotopes multiplied by their percentage abundance.
Thus, in this case:
Atomic mass= m(Ga-69)×%(Ga-69)+X×%(Ga-71)
From the periodic table, atomic mass of Ga is 69.723 amu.
Putting the values,

Thus,

Rearranging,

Therefore, mass of Ga-71 isotope is 70.9246 amu.
<span><span>Law of Conservation of Mass - "The total mass after a chemical reaction is exactly the same as the mass before"</span></span><span>
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Assume 1 liter = 1 kilogram of water = 1000 grams of water.
Part A)
MW of hydrogen is 1.008g/mol, and oxygen is 16.00g/mol.
Find the MW of water by
2*(1.008) + (16.00) = 18.016g/mol.
Convert 1000g H2O to moles :
(1000g H2O)*(1mol H2O / 18.016g H2O) = 55.51 mol
Part B)
Using the answer from part A and Avogadro's number:
(55.51mol)*(6.022*10^23) =
3.343*10^25 molecules.
Hope this is helpful