Question

When mole fractions dont add to 1 we add another C+ to make it 1 for example if we have from c1 until c6 and they dont add to 1 we add c 7+ but then why when we use equations we use the weight of c8 not c7

Answer 1

If a solution contains n number of components, then sum of then mole fractions is equal to 1.

i.e., X₁ + X₂ + X₃ + ..........+ X_n = 1; where X₁ is the mole fraction of first component,

X₂ mole fraction of second component,

X₃ is the mole fraction of 3rd component and
X_n is the mole fraction of nth component. Mole fraction of a component

where n₁ is the no. of moles of 1st component,

n₂ no. of moles of second component and so on.

no. of moles = weight / molecular weight of the component

If 7 components present, then 7 mole fractions will only be there, and sum of that 7 mole fractions will be equal to 1.

If only six mole fractions are available, then X₇ = 1- sum of the six mole fractions.