Calculating Moles of H2X and Mr of H2X in Reaction with NaOH

[mouk92]

(a) An acid, HX, reacts with sodium hydroxide as shown in the equation
below:

H2X(aq) + 2NaOH(aq) -----> 2Na+(aq) + X2-(aq) + 2H2O

A solution of this acid was prepared by dissolving 1.92 g of H2X in water and making the volume up to 250 cm3 in a volumetric flask.

A 25.0 cm3 sample of this solution required 21.70 cm3 of 0.150 moldm-3 aqueous NaOH for complete reaction.

(i) Calculate the number of moles of NaOH in 21.70 cm3 of 0.150 moldm-3 aqeous NaOH.


(ii) Calculate the number of moles of H2X which reacted with this amount of NaOH. Hence, deduce the number of moles of H2X in the 1.92 g sample.

Moles of H2X in 25.0 cm3 of solution .

Moles of H2X in 1.92 g sample


(iii) Calculate the relative molecular mass, Mr, of H2X.




3. The Attempt at a Solution [/b





part i) is 21.7 × 10-3 × 0.150 = 3.255 × 10-3 (mol)
ii) is In 25 cm 3 = (3.255 × 10-3)/2 = 1.63 × 10-3 (mol)

but i don't know how to get the second part, however i know the answer is 1.63 × 10-2

iii) the anwer is 118, and again how do i get to it


thanks.

 

[Nino MMP]

Answer:(i) Number of moles of NaOH in 21.70 cm3 of 0.150 moldm-3 aqueous NaOH = 21.7 cm3 × 0.150 mol/dm3 = 3.255 x 10-3 mol(ii) Number of moles of H2X which reacted with this amount of NaOH = (3.255 x 10-3 mol)/2 = 1.63 x 10-3 molNumber of moles of H2X in 1.92 g sample = (1.92 g/Mr) x (1 mol/ Mr) = 1.63 x 10-2 mol, where Mr is the relative molecular mass of H2X. (iii) Relative molecular mass of H2X = 1.63 x 10-2 mol x Mr = 1.92 g Therefore, Mr = 1.92 g/1.63 x 10-2 mol = 118

 

[Blueshift5]

Hi there,

For part ii), we can use the mole ratio from the balanced equation to determine the number of moles of H2X that reacted with the NaOH. From the equation, we can see that for every 2 moles of NaOH, 1 mole of H2X reacts. Therefore, if we have 1.63 x 10^-3 moles of NaOH, we can calculate the number of moles of H2X by dividing by 2. This gives us 8.15 x 10^-4 moles of H2X.

For part iii), we can use the molar mass of H2X to calculate the relative molecular mass (Mr). We know that the 1.92 g of H2X was dissolved in 250 cm^3 of solution, which is equivalent to 0.25 L. Therefore, the concentration of H2X in the solution is:

(1.92 g / 0.25 L) = 7.68 g/L

To convert this to mol/L, we need to divide by the molar mass of H2X. From the periodic table, we can see that the molar mass of H2X is 36 g/mol. Therefore, the concentration of H2X is:

(7.68 g/L) / (36 g/mol) = 0.213 mol/L

Now, we can use the definition of molarity (M = mol/L) to calculate the moles of H2X in 25 cm^3 of solution:

(0.213 mol/L) * (25 cm^3 / 1000 cm^3) = 5.325 x 10^-3 mol

Finally, to get the relative molecular mass, we can use the definition of molar mass (M = mass/moles). Rearranging this equation, we get:

M = mass/moles = (1.92 g) / (5.325 x 10^-3 mol) = 360 g/mol

This is equivalent to the Mr of H2X, which is 360 g/mol.

I hope this helps and let me know if you have any further questions.