How do scientists determine the number of neutrons in the isotope of an atom?
they find the number of protons.
they divide the atomic mass by two.
they add the number of electrons and protons.
they subtract the atomic number from the atomic mass.

Scientists can calculate the number of neutrons by subtracting the atomic number from the atomic mass. The atomic number of an element is the number of protons which can be found in the nucleus of the element's atoms. ... The number of neutrons is not always equal to the number of protons

Explanation:

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sulfur belongs to group 16 because it can be found in nature whether it is in a free or combined state.

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as given the equation is

2al + 6 hcl > 2 alcl3 + 3 h2(g)

as per equation two moles of aluminum reacts with six moles of hcl to give two moles of alcl3 and three moles of hydrogen gas

given: three moles of al are present and 7 moles of hcl

for three moles of aluminum nine moles of hcl will be required

however as 7 moles of hcl are present so the limiting reagent is hcl

hcl is going to decide the number of moles of product formed

when six moles of hcl are present = 3 moles of h2 are formed

when one mole of hcl is present == 3/6 mole sof h2 will be formed

when 7 moles of hcl are present = 3 x 7/ 6 moles of h2 will be formed = 3.5 moles of h2