that 2.00mL of 0.02 mol/L aqueous sodium sulphic 3 test a 60.0mL sample of water
ID: 715351 • Letter: T
Question
that 2.00mL of 0.02 mol/L aqueous sodium sulphic 3 test a 60.0mL sample of water containing 0.0004 moa phide Suppose Hg2NO3, ions. a. Write the balanced equation 5. b. Determine the limiting reactant Calculate the maximum mass of precipitate formed. Key Question #13 in your own words research describe how a water softening unit (5 marks) 1. 2. 32.12 mL of 10.0 molV/L potassium chromate solution is reacted with 120 2.00 molL lead (I) nitrate solution. The balanced equation is; 2KCrOsua)+Pb(NO3)2(a0)9 2KNOxaa)+ Pb(CrO )26 a. Determine the limiting reactant. (4 marks) b. What is the maximum mass, in grams, of lead (I) chromate that can be produced? (4 marks) 3. How does water dissolve an ionic salt such as sodium chloride? (2 marks) Copyright 0 2008, Durham Continuin 10 of40Explanation / Answer
#13
1. Water softening is the process by which “hard” metals ions like Ca2+ and Mg2+ are exchanged for “soft” ions like Na+ and K+. Hard water contains carbonates and bicarbonates of Ca2+ and Mg2+. Hard water forms scum with soaps and reduce the cleansing action of soaps. Hard water also leads to the formation of limescale in plumbing lines, thereby, reducing the longevity of pipelines.
Conventionally, “hard” water is softened by using “ion-exchange resins”. Ion-exchange resins are organic polymers with anionic functional groups. The charges on the anionic functional groups in ion-exchange resins are balanced by Na+ and K+ ions. When hard water containing salts of Ca2+ and Mg2+ is passed through an ion-exchange resin, the Na+ and K+ ions in the resin are replaced by Ca2+ and Mg2+ ions. This is due to the fact, that Ca2+ and Mg2+ ions contain greater charge density as compared to Na+ and K+ ions and can bind to the anions strongly. The resulting water coming out of the resin contains salts of Na+ and K+. Such water doesn’t form scum with soaps or limestone in pipelines. This is how the process of water softening works.
2a. The balanced chemical equation for the reaction is
K2CrO4 (aq) + Pb(NO3)2 (aq) ----------> 2 KNO3 (aq) + PbCrO4 (s)
As per the stoichiometric equation,
1 mole K2CrO4 = 1 mole Pb(NO3)2 = 1 mole PbCrO4.
Millimoles of K2CrO4 = (volume of K2CrO4 in mL)*(molarity of K2CrO4)
= (32.12 mL)*(10.0 mol/L)
= 321.2 mmole.
Millimoles of Pb(NO3)2 = (volume of Pb(NO3)2 in mL)*(molarity of Pb(NO3)2)
= (120.0 mL)*(2.00 mol/L)
= 240.0 mmole.
Since K2CrO4 and Pb(NO3)2 reacts in 1:1 molar ration, hence, 240.0 mmole of Pb(NO3)2 will react with 240.0 mmole of K2CrO4. Therefore, Pb(NO3)2 is the limiting reactant (ans).
b. The atomic masses are
Pb: 207.2 u
Cr: 51.9961 u
O: 15.999 u
Molar mass of PbCrO4 = (1*207.2 + 1*51.9961 + 4*15.999) g/mol
= 323.1921 g/mol.
As per the stoichiometric equation,
1 mole Pb(NO3)2 = 1 mole PbCrO4.
Therefore,
240.0 mmole Pb(NO3)3 = 240.0 mmole PbCrO4.
Mass of PbCrO4 formed = (moles of PbCrO4)*(molar mass of PbCrO4)
= (240.0 mmole)*(1 mole)/(1000 mmole)*(323.1921 g/mol)
= 77.566 g
77.6 g (ans, correct to three sig. figs).
Note that the formula for potassium chromate is K2CrO4 and not KCrO4.
3. An ionic salt like sodium chloride (NaCl) is made up of oppositely charged ions, viz. positively charged sodium ion (Na+) and negatively charged chloride ion (Cl-). The oppositely charged ions are held together in the salt by electrostatic forces of attraction.
Water (H2O) is a dipole. The hydrogen (H) atom is positively polarized while the oxygen (O) atom is negatively polarized. This is due to the fact that the O atom is more electronegative than the H atom.
When NaCl is dissolved in water, the positively polarized H atom surrounds the negatively charged Cl- ion. The positively charged Na+ ion is surrounded by water molecules and the O atom of the water molecules interacts with the Na+ ion. This interaction between a dipole (H2O) and oppositely charged ions is known as ion-dipole interaction and is predominantly responsible for dissolution of all ionic salts in water.
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