pH 7. A chemist dissolves 0.00053 g of sodium hydroxide in water and dilutes the

Question

pH 7. A chemist dissolves 0.00053 g of sodium hydroxide in water and dilutes the solution to the mark in a 250.00 ml. volumetric flask. A 1 .00 mL sample of this solution is then transferred to a 100.00 mL volumetric flask and diluted to the mark. mactse fiat A. What is the molarity of the sodium hydroxide in the final solution? B. What is the pH of the final solution? 8. A chemist dissolves 0.00491 g of hydrochloric acid in water and dilutes the solution to the mark in a 250.00 mL volumetric flask. A 1.00 mL sample of this solution is then transferred to a 100.00 mL volumetric flask and diluted to the mark. A. What is the molarity of the hydrochloric acid in the final solution? B. What is the pH of the final solution 9. A 12.0 g chunk of zinc is dropped into 200.0 mL of 0.250 M HCI. A. What is the concentration of zinc chloride? B. What is the mass of hydrogen gas produced? C. What is the concentration (molarity) of H in the solution? D. What is the pH of the solution?

Explanation / Answer

mass of Sodium hydroxide taken =0.00053 and dissolved in 250 ml.

mass presnet per ml= 0.00053/250 gm/ml

1ml of this solution is taken, mass present in 1ml= 0.00053*1ml/250ml =0.00053/250 gm/ml

moles of NaOH = mass/molar mass=0.00053/(40*250)= 5.3*10-8 moles

this is diluted to 100ml =0.1L

Molarity= moles/Volume =5.3*10-8/0.1 =5.3*10-7M

pH= -log [H+]

since NaOH dissociates completely [OH-]= 5.3*10-7M, pOH-= -log [OH-]= -log (5.3*10-7)=6.3

pH+ pOH= 14

pH= 14-6.3= 7.7

2. mass of HCl present in 1ml= (0.00491/250)* 1 = 0.00001964 gm

the mass present in 1 ml is diluted to 100ml, moles = mass/molar mass =0.00001964/36.5 gmoles=5.4*10-7,

concentration of HCl= moles/Volume in L= 5.4*10-7 moles/(100/1000L)=5.4*10-6M

since HCl completely ionizes, [H+]= 5.4*10-6, pH= -log [H+]= -log (5.4*10-6)= 5.3

3. moles of HCl in 250ml of 0.25M= Molarity* Volume in L= 0.25*200/1000 =0.05 moles

moles of Zn= mass of zinc/ atomic weight=12/65.38=0.1835

The reaction between Zn and HCl is Zn+ 2HCl ----->ZnCl2+ H2

Theoretial molar ratio of Zn : HCl= 1:2, Actual molar ratio of Zn :HCl =0.1835:0.05= 0.1835/0.1835:0.05/0.1835= 1: 0.27

So HCl is limiting reactant. Since requires is 2 while supplied is only 0.27 for 1 mole of Zn. So all the HCl reacts.

2 mole of HCl reacts to give 1 mole of ZnCl2, 0.05 moles of HCl forms =0.05/2=0.025 moles of ZnCl2. This is present in 250ml, so concentration of ZnCl2= 0.025/(250/1000)=0.1M

moles of H2 prpduced= 0.5* moles of HCl used =0.5*0.05=0.025

pHmass of H2 produced =moles* molar mass =0.025*2=0.05 gm

HCl being strong acid ionizes completely, [H+]=0.25M

pH= -log [H+]= -log (0.25)= 0.60

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