Question

(a) A 0.481 gram sample of an unknown acid (HX) required 28.95 mL of 0.2103 M NaOH for neutralization to a phenolphthalein endpoint. What is the molar mass of the acid?

(b) If the unknown acid had the formula H2X (2 is subscript) could this experiment be used to determine molar mass? Explain.

Answer 1

since the acid is monoprotic (only 1 H+), you know that the moles of acid and moles of base used must be equal. The volume and concentration of base used will get you the moles of base used. The moles of acid used must be the same.

the mass of the unknown acid divided by the moles calculated will equal the molar mass.

if the ratio were 2:1, you could still use it to find molar mass, but it would have to be a different acid than the HX from above.

Answer 2

ok I didn't know that H+ indicated that the acid and base had the same number of moles. I know how to get to the molar mass from here, but I'm confused about your last paragraph (relating to part b?)

Answer 3

the formula HX makes the acid monoprotic. the mole ratio of H+ to OH- is 1:1 in order to make water, which is what all neutralizations do.

for part B, you still have a mole ratio, but it's not a 1:1 ratio anymore, it's a 2:1 ratio

Answer 4

its a 2:1 ratio because you have to add a coefficient to balance the chemical equation? How do you know its 2:1 if you don't know what X is?

Answer 5

because the part that really matters in a neutralization is the H+ from the acid, and the OH from the base

Answer 6

im getting:

HX + NaOH -> NaX + H2O (first part)

H2X + NaOH -> NaX + H3O (second)

Answer 7

your first equation is correct, but your second needs work

Answer 8

\[HX + NaOH \rightarrow NaX + H_2O\]watch how i split these ions up\[(H^{+1})(X^{-1}) + (Na^{+1})(OH^{-1})\rightarrow (H^{+1})(OH^{-1} + (Na^{+1})(X^{-1})\]

Answer 9

do you see how the H+ from the acid and the OH- from the base always combine 1:1 to form water?

Answer 10

yeah i see that

Answer 11

their ratio is 1:1 because their charges "fit" together to make a neutral compound

Answer 12

now if the acid has 2 H+ pieces, each one makes a water molecule with a differnt OH-

Answer 13

\[H_2X + 2NaOH \rightarrow 2H_2O + Na_2X\]

Answer 14

this is what makes the mole ratio 2:1. the charges of H+ and OH haven't changed, but you now have MORE PIECES that need to be used

Answer 15

ohhh so the X changed charge because there are 2 H's which means you need 2 Na's and ... rest of formula...

thanks i have to get to class but ill look this over later tonight..

Thanks you so much.

Answer 16

YW