Molarity of oncentated nitric acid

[geffman1]

Homework Statement

hey guys, i got a problem. "concentated nitric acid is 70.7% HNO3 (molar mass=63g) and 29.3% H20 by mass. the density of the solution is 1.42gcm. what is the molarity.

Homework Equations

i know i should rearrange the formula d=m/v but i can't work out the mass becuase how do i know how many moles of HNO3 do i have? i tried writing empricial formula but I am a bit lost. any help would be greatlyfull apprecitated.

The Attempt at a Solution

P.s I've got an assesment and i have a few worked questions, if anyone would like to check one or 2 answers please let me know. thanks

 

[Redbelly98]

Start by working with those mass-% numbers. You may assume any amount of HNO₃ and H₂O, as long as they are in the right proportion. So to make life simple, use 70.7g of HNO₃ and 29.3g of H₂O.

 

[geffman1]

yeh i did that but when i worked it out i get 1.122 moles of HNO3 and 1.623 moles of H20. should i just mulitiple both by 2?

 

[Redbelly98]

Not sure what good multiplying by 2 does.

At any rate, you now have how many moles of HN03. Given the density 1.42gcm (is that 1.42 g/cm³?), you can figure out the volume of the solution.

 

[geffman1]

so if i times 70.7 grams by 1.122 that should give me the mass of HNO3

 

[geffman1]

oh soz, yeh it is cm^3

 

[Redbelly98]

Uh, no, the mass of HNO3 is 70.7 grams. 70.7g x 1.122mol would be meaningless here.

The total mass of the solution is 100.0 grams. Use the density to get the volume of the solution.

 

[geffman1]

o true. so it would work out v=m/d =100/1.42=70.422. then molarity would equal moles/volume which would equal 1.122/70.422=0.0159

 

[geffman1]

thansk mate your a ledgend.

 

[Redbelly98]

You are very very close, hang in there.

Watch those volume units:
What are the units for the v=70.4 you calculated?
What volume units are used to calculate molarity?