okay let's try some calculations regarding molarity. the first question
asks a solution contains 50 point 0 grams of sucrose which has the formulas
C12H22O11 and has a volume of 210 milliliters.
what is its concentration? second question asks what mass of sucrose is
contained in 750 milliliters of a 0.250 molar solution? and the third question
asks if 10.0 grams of sucrose are dissolved in water and the resulting
solution is 2.3 molar what is the volume of the solution? so if
you're not sure how to approach these check out my tutorial on solutions and
molarity and that'll run you through the strategy for these kinds of questions
and when you're ready to give it a try.
so let's take a look at this first one what is the data that we have well we
have 50 point zero grams of sucrose but if we're looking at concentration if
we're trying to get concentration we know that that is equal to moles per
liter so we don't want grams we want moles so let's take our 50 grams and
calculate moles so if we add up 12 carbon atoms 22 hydrogen atoms 11 oxygen
atoms we're gonna get to 342.3 grams as the molar mass of sucrose so
let's do 50.0 grams times 1 mole over 342.3 grams and that means
we have 0.146 moles of sucrose. now to get the concentration we
just need to put moles over liters because concentration is moles per liter
and so we have 0.146 moles as we just got and then liters we have 210
milliliters so let's just call that zero point two one zero liters and then
we can plug that into the calculator and we get zero
point six nine six molar that is the concentration of this solution of
sucrose now moving on to the second one we're looking for a mass and we're given
a concentration and a volume so using concentration we can get moles and
hopefully from moles we can get to mass so let's set this up this way we know
that concentration or molarity is mole's per liter so let's put concentration
equals moles over liters and plug in what we know we have zero point two five
zero molar that's the concentration that is going to be equal to the moles
however many moles we have over zero point seven five zero liters remember we
want to convert milliliters into liters in order to do this and get the right
math so if we solve for n we end up getting zero point one eight eight moles
that's how many moles of sucrose we have in here and then luckily we already
calculated the molar mass of sucrose so let's just take 0.188 moles of sucrose
times 342.3 grams per mole and that gives us 64.4 grams of
sucrose in this solution. okay let's look at the last one so we have ten grams of
sucrose and we dissolve that in water and the resulting solution is 2.3 molar
so we want to know what the volume of the solution is so once again if we have
mass we're gonna need moles so let's just go ahead and convert that right off
the bat. ten grams times one mole over three hundred forty two point three
grams that means we have zero point zero to nine two moles of sucrose now we're
ready to do a calculation involving the concentration we know that concentration
equals moles over liters right moles per liter so let's take 2.3 molar that's the
concentration that we know equals zero point zero two nine two moles over some
volume and liters so now we can solve for volume so let's solve for volume in
this equation and that is going to be zero point
zero one to seven liters which is equal to twelve point seven milliliters and
that is the volume of this sucrose solution.