Hi. Thanks for asking this question. Before answering, I have a tip that is helpful for chemistry classes as well as for professional jobs like nursing, medicine and engineering: To not lose the decimal point, put a zero in front of it:
Here's the modified question and this suggestion will help prevent errors.
A lab team places... [0.0250g] ...of an unknown solid acid in an Erlenmeyer flask. They neutralize the solid acid with 15.0ml of
[0.210 M Na(OH)(aq)]. One mole of the acid reacts with one mole of the base.
Calculate the molar mass of the unknown solid acid
Calculate the molar mass of the unknown acid."
Now, do you know all the words in the question? Can you define them:
A lot of the answers in chemistry (and physics, for that matter) come from the definitions, so, lets look at a key definition:
In chemistry, the molar mass is a physical property. It is defined as the mass of a given substance (chemical element or chemical compound) divided by its amount of substance....molar masses have units (in this case, grams/mole)... The molar mass of a compound is given by the sum of the standard atomic mass of the atoms which form the compound multiplied by the molar mass constant, Mµ:
M(NaCl) = [22.989 769 28(2) + 35.453(2)] × 1 g/mol = 58.443(2) g/mol [http://en.wikipedia.org/wiki/Molar_mass, accessed May 19, 2013]
You see from the definition you need to use g/mol to find your answers.
Now, what about that capital "M?"
molarity - M → moles per liter solution
molality - m → moles per kilogram solvent
[http://chemistry.about.com/od/chemistryterminology/a/What-Is-The-Difference-Between-Molarity-And-Molality.htm, May 19, 2013]
From the above definitions, you can see tat a, "Concentration of 0.210 M,"tells you you have
(How many) ______ moles per liter, and
15.0ml of [0.210 M Na(OH)(aq)]. One mole of the acid reacts with one mole of the base. [from the question],
which tells you, you've have (how many)_________ milliliters and which means you have (how many) ___"A"____ liters?
The question further states,
One mole of the acid reacts with one mole of the base.
How can you use that information? (Or can you?) Aha! It gives you a ratio. What ratio? How do you use it? One mole of Na(OH) reacts with one mole of the acid, so the two are in a ______:_______ ratio. Therefore, if it is 1:1, or 2:1, or 3:1, or 1:3 you can figure out how many moles you have of each. What ratio gives you the same number of moles of Na(OH) as you have moles of the acid?
You are given the number of moles of solute and the number of liters of solution and the ratio.
Calculate the number of moles of Na(OH). Let's see...
0.21 moles = (x moles) or 0.21moles (0.15 liter) =
x moles or __________moles of NaOH
liter 0.15 liter liter
Hints... Cancel out the units and see if you get the right units for the answers.
Don't forget "significant digits."
What does, "aqueous," tell you and do you need to use this information in your calculations?
Can you figure out the empirical formula by calculating it from moles? There are 6.02 x 1023 amu (atomic mass units) in a gram of a substance. See Avogadro's number if you have questions on that. Do you need this information? Why or why not?
You now know that ___ moles (the, "A," you calculated) of this acid weighs
0.025 grams (you got from the question), but you want to find the molar mass.
Define molar mass:
Definition: The mass of one mole of a substance, usually expressed in grams or kilograms.
GMM O2 = 32.0 g or KMM O2 = 0.032 kg [http://chemistry.about.com/od/dictionariesglossaries/g/defmolmas.htm, accessed May 19, 2013]
So, set up a proportion paying attention to units.
x___g_ = 1 mole , so, x = _____________ grams, which is the molar mass of the acid.
0.025g 0.00315 mole,
Once you have answered the assigned question, you might also want to compare and contrast M with m for your own learning enhancement. This further study will help you lock what you have learned into your brain so you remember it for tests and for your job.
The molecular mass (m) is the mass of a given molecule: it is measured in [atomic mass unit]]s (u) or daltons (Da), where 1 u = 1 Da = 1.660 538 782(83)×10−27 kg). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The molar mass is a measure of the average molecular mass of all the molecules in a sample, and is usually the more appropriate measure when dealing with macroscopic (weighable) quantities of a substance. [http://en.wikipedia.org/wiki/Molar_mass#Molar_masses_of_compounds, accessed May 19, 2013]
Review all the definitions and double check your answers. Try some similar problems and see if you can get them. Have fun with it. Play with the numbers and the units. Try different ratios. Let me know how it works for you to use definitions and units in problem solving. Oh, and practice your times tables.
Note: If the doctors and nurses working with my grandfather had known that tip, he wouldn't have received a lethal dose of his medicine by mistake. He was given medicine a factor of 10 times too strong. So, don't think that the details on this question don't matter, they do. You can prevent medicine errors and other errors. In so doing, you will save lives.