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Molarity

The most common way to express solution concentration is molarity (M), which is defined as the amount of solute in moles divided by the volume of solution in liters: M = moles of solute/liters of solution. A solution that is 1.00 molar (written 1.00 M) contains 1.00 mole of solute for every liter of solution. Created by Sal Khan.

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  • piceratops tree style avatar for user ŇØŦ€ŞŁΔ¥€Ř
    Is molarity the same as concentration?
    (5 votes)
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    • leaf red style avatar for user Richard
      Concentration in general is a measure of how abundant a component is in a mixture. And molarity is one way to measure concentration and is the most commonly used in chemistry. But there are other ways to measure it too like molality, weight %, ppm (parts per million), etc. So for most chemistry purposes, molarity is the same as concentration since it's the primary unit we use.

      Hope that helps.
      (20 votes)
  • blobby green style avatar for user grantbailey
    Is there such thing as a maximum molarity a solution could attain? And why is this?
    (5 votes)
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    • leaf red style avatar for user Richard
      Yes, a solution can become saturated. When you dissolve a solute into a solvent you create a solution where the solute has a concentration measured in molarity. If you can no longer dissolve any more solute into the solvent, your solution has become saturated therefore essentially capping the molarity of the solute. When you dissolve something like salt, sodium chloride or NaCl, into water, the water molecules surround each individual sodium and chloride ion thereby turning them into aqueous ions and increasing the molarity. If you keep adding more salt you need more and more water molecules to surround those ions to actually make them dissolve. Eventually you run out of free water molecules to surround the ions and the salt can no longer dissolve so it stays in its unaltered solid. The solid form does not add to the molarity since molarity only measures aqueous species in solution. You would observe solid salt particles just falling to the bottom of the water in a saturated solution. Hope that helps.
      (12 votes)
  • primosaur seedling style avatar for user Agnes Gwanbobmuga
    At , how do I figure out the molar mass
    (3 votes)
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  • duskpin seedling style avatar for user Kat
    How do you find the mass of a solute?
    (1 vote)
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    • leaf red style avatar for user Richard
      I'm going to assume that you mean 'how do you find the mass of the solute if you're only given molarity' since that's the title of the video.

      So molarity is defined as moles of solute in moles divided by volume of solution in liters. Or as an equation M = n/V, where M is molarity, n is moles of solute, and V is volume. If you multiply the equation by volume you have a way to solve for number of moles, or MV = n. So basically you can multiply the molarity by volume to find the number of moles of solute. Then you can use the molar mass of the solute to convert moles of solute into grams of solute, or the mass.

      Hope that helps.
      (6 votes)
  • blobby green style avatar for user algifarihaikal123
    i still dont get a clear idea of sig fig. can anybody explain it please...
    (1 vote)
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    • female robot ada style avatar for user SarahLDeMonia
      Hi there! Significant figures can be a bit tricky but they become much easier with examples. For example, the numbers 0.00451, 2.30, and 4,500.
      Let's look at 0.00451. Because of the leading zeroes rule, you would start counting significant digits starting at the '4'. Thus, this number has 3 significant digits. Once again, this is because leading zeroes are not significant.
      As for 2.30, you notice there is a zero but it is after the decimal place. Because it was deliberately included by the researcher it is significant. Thus, we would say that 2.30 has 3 significant digits.
      As for the last example, 4,500, you notice there are zeroes that follow the five but because there is no decimal place we consider these non-significant. A neat trick is to see how the number looks in scientific notation. 4,500 can be written as 4.5 x 10^3. In other words, the zeroes are not needed to convey the significance of the number. If I had 4,500.0, however, the researcher deliberately was trying to say that the tenths place was significant and so they left the decimal. In 4,500.0, then, there would be 5 significant digits.
      I found a response by Chris Martin on Quora which does a better job of explaining this concept:
      "Because they are assumed to be rounded zeroes instead of measured or calculated ones. The existence of a decimal point indicates that the zeroes are part of the actual measured or calculated value."
      I hope this helps you! Let me know if you have any more questions!
      (6 votes)
  • duskpin tree style avatar for user J.L.MIKEY
    If I have 3.5 moles of lithium fluoride and 4.0 liters of solution do I need to find the molar mass?
    (2 votes)
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  • female robot amelia style avatar for user darda.clawson
    how do you find the volume of a solution like this A 25 mL sample of 0.100 M HNO3 completely reacts with NaOH according to this equation:
    HNO3(aq) + NaOH (aq) ⟶ NaNO3(aq) + H2O(l)

    What volume of 0.0500 M NaOH solution is required to completely react with the HNO3
    (2 votes)
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    • leaf red style avatar for user Richard
      This is an acid-base neutralization reaction, where the acid, nitric acid, is neutralizing the base, sodium hydroxide. The key point here is that to completely have all the nitric acid reaction with the sodium hydroxide so that there isn’t any acid or base left, we have to have the same moles of nitric acid and sodium hydroxide.

      So you would need to find how many moles are in 25 mL of 0.100 M nitric acid, and that’ll be the same amount of moles of sodium hydroxide needed. You can do with the molarity formula, molarity (M) = moles of solute (moles)/volume of solution (Liters). Once you have the moles of sodium hydroxide, you can calculate the volume of sodium hydroxide needed using the concentration, 0.0500 M.

      Hope that helps.
      (2 votes)
  • blobby green style avatar for user lankeshwariwijeckoon
    how to calculate molarity
    (2 votes)
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  • blobby green style avatar for user Darlene Angelica
    I have a question, if for example we are given the volume of solution and Molarity. But asked for the grams of solute , can we multiply Molarity and volume given to look for moles of the solute first. Then, calculate molar mass , and then use moles & molar mass to find the grams solute?
    Ex : how many gr Ethanol are in a 0.05 L of a 0.25 M solution of ethanol?
    (2 votes)
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  • blobby green style avatar for user s1763645
    Is molarity the same as concentration?
    (1 vote)
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    • old spice man green style avatar for user Elijah Daniels
      Molarity is one of a few ways to describe concentration. It's moles of solute per liter of solution. There are 3 main other ways to describe concentration, such as mass percent/percent by mass (percent mass per volume), percent by volume, and molality (yes this is spelled correctly - it's moles of solute per kilogram of solvent - notice how different this is from molarity).
      (2 votes)

Video transcript

- [Instructor] In this video, we're gonna talk about one of the most common ways to measure solute concentration in a solution. And that is molarity. And molarity is defined as the number of moles of solute, the thing that we are dissolving in a solvent, that divided by the liters of solution. So let's just do an example and see if we can calculate the molarity of a solution. So let's say that I have, this is a container here. And I'm going to dissolve some sodium sulfate in water. So sodium sulfate is the solute and water is the solvent. Together, they give us this solution. And let's say we have a total, the total volume of solution is 250 milliliters of solution. And that solution is made up of just to give ourselves a bit of a refresher. We have the solvent, which is H2O, it is water in this situation. And you might say, do we have 250 milliliters of water? And the answer would be not quite because the 250 milliliters that's the volume of the water plus the sodium sulfate. And so we're gonna have some sodium sulfate in here. And let's say we know that we have 35.5 grams of sodium sulfate. That is the formula for sodium sulfate. And so given this information, how do we figure out molarity? Well, the first thing you might say is okay, I know the number of grams of sodium sulfate. I need to figure out the number of moles. And to figure out the number of moles, you'd have to look at the molar mass. You could figure that out from a periodic table of elements. But just to speed us along, I will help you out a little bit here. The molar mass sodium sulfate, molar mass is 142.04 grams per mole. So given everything I've now told you, see if you can pause this video and figure out the molarity of this solution. What's the molarity of the sodium sulfate in this solution? All right, now let's work through this together. So first, we wanna figure out the number of moles of solute. So we can start with the mass of solute that we have right over here. So we have 35.5 grams of sodium sulfate. And now if we wanna figure out the number of moles, see I'm gonna multiply this times something that would cancel out the grams. And so I don't want grams per mole. I want moles per gram. And so I could write this. I could multiply this times for every one mole of sodium sulfate, NA2SO4, we have 142.04. I'll write this way, grams of sodium sulfate. And you can see very clearly that that will cancel with that and we're left with moles of sodium sulfate. So we'll get a calculator out in a second and just take 35.5 and divide that by 142.04. And then to figure out molarity, we wanna divide by the liters of solution. So up here we have a calculation for number of moles and then the liters have solution, 250 milliliters is the same thing as 0.250 liters of our solution. And now we can just use our calculator to figure out what this is. 35.5 divided by 142.04 equals that, and then we divide that by 0.25. I could just throw a zero in there. And then that gets us that right over there. And then we can think about how many significant figures we have. So we have three over here. We have one, two, three, four, five over here. We have three over here. So, I would say that we have three significant figures. And so we would round this right most or this nine right over here. Well, if we round that up, we get 1.00, if we were to go to three significant figures. So that gets us 1.00. And then you might say what is the units here? And what people will normally say is this is 1.00 molar. When you see this capital M right over here, that is the unit for more molarity but they're really talking about the number of moles of solute per liter of solution. And we are done.