Complete the sentence to explain why the atomic weight of carbon is reported as 12.011 in the table of elements and the periodic table in the front inside cover of the textbook.

The average mass of a carbon atom 12.011, is the average of the masses of all the isotopes ¹²C and ¹³C

The elements in nature have several types of isotopes

Isotopes are atoms whose atomic number has the same number of protons while still having a different number of neutrons.

So Isotopes are elements that have the same Atomic Number (Proton)

Atomic mass is the average atomic mass of all its isotopes

In determining the mass of an atom, as a standard is the mass of 1 carbon-12 atom whose mass is 12 amu

So the atomic mass is obtained is the mass of the atom relative to the 12th carbon atom

An atomic mass unit = amu is a relative atomic mass of 1/12 the mass of an atom of carbon-12.

The 'amu' unit has now been replaced with a unit of 'u' only

The natural abundance of ¹⁴C, ¹⁰C and ¹¹C is so small that we use the isotopes of ¹²C and ¹³C to calculate the average mass of a carbon atom

The average atomic mass is the average of the masses of all

the isotopes

Average atomic mass X = mass isotope 1. % + isotope mass 2.%

The natural abundance of ¹³C is 1.11%, and the natural abundance of ¹²C is 98.89%

The mass of ¹²C is 12 amu.

the mass of ¹³C is 13.003355 amu

so that:

average atomic mass C = (0.9889 x 12 amu) + (0.0111 x 13.00335 amu)

 = 11.8668 + 0.1443

 = 12,011 amu

element  X  

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the chemical symbols

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two stable isotopes of bromine

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Keywords : the isotope, C, atomic number , mass number

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