A 13.1 g sample of an aqueous solution of hydrobromic acid contains an unknown a

Question

A 13.1 g sample of an aqueous solution of hydrobromic acid contains an unknown amount of the acid.
If 21.3 mL of 1.99 M sodium hydroxide are required to neutralize the hydrobromic acid, what is the percent by mass of hydrobromic acid in the mixture?

? % by mass

In the laboratory a student combines 39.3 mL of a 0.381 M nickel(II) fluoride solution with 17.6 mL of a 0.470 M ammonium fluoridesolution.

What is the final concentration of fluoride anion ?

M

Write a net ionic equation for the reaction that occurs when aqueous solutions of acetic acid and ammonia are combined.

Explanation / Answer

1.

NaOH (aq.) + HBr (aq.) -----------> NaBr (aq.) + H2O (l)

Moles of NaOH = molarity * volume of solution in L = 1.99 * 21.3 / 1000 = 0.0424 mol of NaOH

From the balanced equation,

1 mol of NaOH needs 1 mol of HBr

Then, 0.0424 mol of NaOH needs 0.0424 mol of HBr

The, mass of HBr = moles * molar mass = 0.0424 * 81.0 = 3.43 g.

% by mass of HBr = mass of HBr * 100 / Mass of sample = 3.43 * 100 / 13.1 = 26.2 %

2.

NiF2 (aq.) ----------> Ni2+ (aq.) + 2 F- (aq.)

NH4F (aq.) ---------> NH4+ (aq.) + F- (aq.)

Total [F-] = [(2 * 39.3 * 0.381) + (17.6 * 0.470)] / (39.3 + 17.6) = 0.672 M

3.

Molecular equation:

CH3COOH (aq.) + NH3 (aq.) ------------> CH3COONH4 (aq.)

Net ionic equation is,

CH3COOH (aq.) + NH3 (aq.) ------------> CH3COO- (aq.) + NH4+ (aq.)

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