Plzz help!!! Chemistry

Question

One cup of fresh orange juice contains 126mg of ascorbic acid(Vitamin C, C6H8O6).

Given that one cup=240.8mL, calculate the molarity of vitamin C in orange juice. Express your answer to an appropriate number of sig figs with the appropriate units.

I got M= 2973M and it's giving me the same description:recall that molarity has units of moles per liter(mol/L). For the unit section, I tried putting M, mol/L, moles/liter and it's giving me the same thing..what's wrong?

Answer 1

Hi, you seem to have some calculation error and it might have been converting mg to grams or mL to L. It may be easier to do it the long way so that it is simple and more error-proof.

Firstly, convert 126mg of ascorbic acid (the solute) to grams:

126mg × 1g/1000mg = .126g of ascorbic acid

Convert these grams into moles using molar mass of 176.14g/mol:

.126g × 1mol/176.14g = 7.1534 × 10^-4 mol of ascorbic acid

Then convert the amount of solvent (orange juice) into liters:

240.8mL × 1L/1000mL = .2408L of orange juice

Now use your simple formula of molarity, moles/liters:

7.1534 × 10^-4mol ⁄ .2408L = 2.97 × 10^-3 M

Note: because the question requires to use appropriate sig figs, your final answer must only have 3. This is because the lowest amount of sig figs in the starting numbers is 3.

Hope this helps!

Answer 2

Given: volume is 240.8 mL which is 0.2408 L

mass of C₆H₈O₆ is 126 mg which is 0.126 g

Asked for concentration in molarity (mol/L)

You must convert the mass to moles using the molar mass of C₆H₈O₆

Molar mass of C₆H₈O₆ is (6x12.011g) + (8x1.008g) + (6x16.0g) = 176.13 g/mol

Conversion: 0.126 g x (^{1 mol C6H8O6}/_{176.13 g C6H8O6}) = 0.000715 mol C₆H₈O₆ (three sig figs)

Concentration (in molarity) = ^{0.000715 mol C6H8O6} / _{0.2408 L} = 0.00297 M

With respect to sig figs, mass is given with 3 sig figs so mol should have 3 sig figs, and volume is given with 4 sig figs; the rule for doing multiplication/division is answer can have no more than the multiplier with the least number of sig figs. In this problem, it is the mass (or mol) that requires the answer to have three sig figs.

Answer 3

Hi Laura,

Let me give it a try:

(126 mg / 240.8 mL) * (1 g / 1000 mg) * (1 mol Vitamin C / 176.14 g) * (1000 ml / L) = 0.0029 mol/L or 0.0029 M.

We came out with a similar number, just at different scales. That possibly means you had the unit conversions flipped so the number was multiplied when it should have been divided. The easiest way to setup these problems is to see what units your are starting with, then what you need to end with. Once you put your conversions in the middle, you should be able to cross cancel (see above) all of the units except for the ones you expect to end with. Once you have that verified, calculate away!

Another check is to think about the plausibility of your answer and back convert. For example, a highly concentrated solution of HCl is 12 M. Therefore, something 2,973 M would be much much higher. If you were to convert back from this number you would expect to end up with the starting value. However, you would find that you actually get 523,664 g/mL which is some very Vitamin C rich and likely pretty chunky OJ!

Hope this helps!

Cheers,

Andrew