Question

Determinati contentratia molara a urmatoarelor solutii:
a)in 200 mL solutie sunt dizolvati 20 mmoli NaOH.
b) in 100 mL solutie sunt dizolvate 8g NaOH.
c)se amesteca 200 g solutie de NaOH 20% (densitate=1,225g/cm3)cu 36,74 mL apa.

Answer 1

Notez "*" inmultire si "/" impartire:
Cm=concentratia molara,mol/L
n=nr moli
m=masa,g
M=masa molecular,g/mol
Vs=volumul solutie, L
ms=masa solutiei,g
Cp=concentratia procentuala(procente masice), %
md=masa substantei dizolvate,g
ro=densitate
V=Volum, L
roH2O=1g/cm3


a) MNaOH=40g/mol
n=m/M ==>m=n*M=20*40=800mg=0.8g
Vs=200ml=0.2L
Cm=md/M*Vs=0.8/40*0.2=0.1M
sau
Cm=n/Vs(L)=20/0.2=100ml/mol=0.1mol/L

b) Vs=100ml=0.1L
md=8g
Cm=md/M*Vs=8/40*0.1=8/4=2mol/L

c) ms1=200g
    Cp=20%
    VH2O=36.74mL
     ro=1.225g/cm3

Din Cp ==>md: ===>Cp=md/ms*100 =>md=Cp*ms/100=20*200/100=40g NaOH

Din ro ==>Vs1: ro=ms/Vs =>Vs1=ms/ro=200g/1.225g/cm3=163.265cm3

Din VH2O==>mH2O: VH2O=mH2O/roH2O =>mH2O=VH2O*roH2O=36.74*1=36.74g H2O
msf=ms1+mH2O=200+36.74=236.74g
Vf=Vs1+VH2O=163.265+36.74=200cm3=0.2L
Cm=md/M*Vsf=40/40*0.2=5mol/L=5M