moles question (2 Viewers)

[clintmyster]

1)how many moles of sodium are there in a 175g packet of table salt, NaCl?

2)one litre of chlorine gas weighs 3.214g when measured at STP. use this info to calculate the atomic mass of the element chlorine. is this result the same as is quoted on the periodic table? suggest a reason for any difference

 

[me121]

1)

number of moles = mass / molar mass
= 175 / (atomic mass of Na + atomic mass of Cl)

(remember number of moles is a measure of how many particles. however because this number is large we say that 1 mole is the number of particles in some mass of some substance)

 

[lolokay]

2)one litre of chlorine gas weighs 3.214g when measured at STP. use this info to calculate the atomic mass of the element chlorine. is this result the same as is quoted on the periodic table? suggest a reason for any difference

a mole of a gas at STP has a volume of 22.71L.

molar mass/mass = molar volume/volume

this gives you a molar mass of 72.99 for Chlorine gas, however as Cl gas is diatomic (Cl2) you divide by 2, = 36.49 atomic mass.

Reasons for the different atomic mass (Cl's atomic mass is 35.45) that I can think of would be - the laws apply for ideal gases, so actual gases would be a bit different; there may have been an abundance of Cl-37 isotope; error in measurement.

 

[xiao1985]

Re^

Non-ideal gas sounds about right. Ideal gas assumes no interactions between gas molecules, but in the practical sense, there's always (at least) dispersion forces.