WEBVTT
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A large balloon has a volume of 1.767 cubic feet.
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It is filled with helium, which has a density of 0.010238 pounds per cubic foot.
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What is the mass, in pounds and correct to three decimal places, of the helium in the balloon?
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So the first thing we need to decide is what type of question is this.
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Well, if we take a look, we’ve got volume, density, and mass.
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And what this tells us is that we’re going to use the mass-volume-density triangle.
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And this enables us to choose the correct formula to work out the mass, volume, or density given the other two.
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So let’s see what we’re looking for.
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First of all, we’ll write down the information we’ve got.
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So we’re told that the volume of the balloon is 1.767.
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And that’s cubic feet.
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We’re told the density of helium is 0.010238 pounds per cubic foot.
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And we’re not told the mass because this is what we’re looking for.
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Well, if the mass is what we’re looking for, we look at the other two, so density and volume.
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Because they’re next to each other, it means we’re gonna multiply them together.
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So therefore, the formula we’re going to use is that mass is equal to density multiplied by volume.
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And it’s always worth checking out the units to make sure that you have the correct formula.
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So if you’re not sure that you’ve got the correct formula, you can look at units.
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So let’s think that the mass is pounds.
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And it’s gonna be equal to the density.
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Well, we’re told that the density is pounds per cubic foot.
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And then, we’ve got this multiplied by the volume.
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And we’re told that the volume is in cubic feet.
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Well, if we multiply pounds per cubic feet by cubic feet, then what we’re gonna have is the cubic feet is gonna cancel.
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And we’re left with pounds.
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And this is what we want the mass to be in.
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Okay, so now let’s substitute our values into the formula.
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So the mass is gonna be equal to 0.010238 multiplied by 1.767 cause that’s the density multiplied by the volume.
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So that’s gonna be equal to 0.01809054, etc.
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So have we finished here?
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Well no, cause we need to check the accuracy.
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And the question has asked us to leave it correct to three decimal places.
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So therefore, the mass of helium is gonna be equal to 0.018 pounds to three decimal places.
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And the way I rounded that was I looked at the third decimal place, which is where the eight is.
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And then I take a look at the digit or number to the right of it.
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And this is the deciding number.
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Well, if this is less than five, then the number remain as it is.
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If it’s five or greater, then we round it up.
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Well, as it’s zero, that means the eight remained as it was.
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So we said that the answer was 0.018 pounds to three decimal places.