The label on the bottle of drinking water contained the following information: (

Question

The label on the bottle of drinking water contained the following information: (a) Calculate the concentrations in mg/L of H+ and OH . How significant are these values compared to the (a) concentrations of the other ions? The term ''hardness" refers to the sum of the concentrations of the divalent cations Ca+ and Mg+. Hardness is often expressed as mg L of CaC03. because calcium carbonate is the mineral that often results in hardness. What is die hardness of this bottled water, expressed as mg/L of CaC03? According to one text book I looked at, "soft" water has hardness of less than 75 mg/L as CaC03: 75-150 mg/L is moderately hard: 150- 300 mg/L is hard: and > 300 mg/L is very hard. Where does this water fall in this scale?

Explanation / Answer

a) pH = 7.8   

-log[H+] = 7.8 [H+] = 10-7.8

pOH =14- 7.8   

-log[OH-] = 6.2 [OH-] = 10-6.2

COMPARE TO CONCENTRATION OF OTHER IONS THESE ARE NEGLIGABLE

b) Ca2+ give concentration as = 78 mg/ml = 78/40 = 1.95 milli moles ........ (40 is atomic weight of Ca

and Mg 2+    give concentration as = 24 mg/ml = 24/24 = 1 milli moles   ....... (24 is atomic weight of Mg)

total number of mole of Ca and Mg = 1.95+1 = 2.95milli moles per ml = 2.95/1000 milli moles per litre........................ = 2.95*10-3 milli moles/litre is hardness =  2.95*10-6 moles/litre is hardness

since we need hardness in the form of CaCO3 only so we have to take 1.95 milli moles only because we have to consider Ca only. so we have to know the weight of 1.95milli mole of CaCO3 = 1.95*10-3 * 100 ................(100g is molecular weight of CaCO3) = 0.195g/litre = 195mg/L

SINCE OUR VALUE IS IN BETWEEN 150 AND 300 our water sample is hard

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