Density and Percent Error

You’ll love it!

What is this Density Business?

• Density – a ratio of mass per volume.

• It is how we measure how compact or “squished together” something is.

Less density

More density

Which is more compact?

So which has a larger density?

Now with MATH!

Density = mass ÷ volume

What is the densityof something with amass of 15g and a volume of 5.2mL?

m

D VD = 15g = 2.9g/mL

5.2mL

UNITS!!

• I’m dividing mass (g) by volume (mL), and neither of those units cancel out!!

• So what do I do? KEEP THEM!

• The unit for density is g/mL or g/cm3. These are the same

thing…

You Do Two!

• What is the density of something with a mass of 8.3g and a volume of 21.4mL?

• What is the density of an object with a volume of 30mL and a mass of 60g?

D = 8.3g ÷ 21.4mL = 0.39g/mL

D = 60g ÷ 30mL = 2g/mL

What If I Know Density??

• What is the mass of something with a volume of 25mL and a density of 2g/mL?

• Cover up the one you NEED on the triangle. Then solve!

m = D x V = 2g x 25mL mL

m

D V= 50g

You Do Two

• What is the volume of an object with a density of 0.8g/mL and a mass of 12g?

• What is the mass of an object with a volume of 12cm3 and a density of 1.8g/cm3?

m

D V

V = m = 12g = 15mL 20mL

D 0.8g/mL

m = D x V = 1.8g x 12cm3 = 22g

cm3

Percent Error

• Percent Error - how far off your measurement is from the true value.

• % error =   | your result - true value |    x  100 %                                true value

Absolute value:

Pretend it’s positive even if it’s not

x 100%

x 100 in your calculator and write a % at the end of your answer.

See it in Action!

• You get a mass of 13.5g in lab.• The true value is 15.2g.• What is the percent error?

% error =   | your result - true value |    x  100 %                               true value

= |13.5 – 15.2| x 100% 15.2

= 11.18%

= 1.7 x 100% 15.2

You Do Two

• You get 14.8mL, but the true value is 10.4mL. What is the % error?

• You get 58cm3, but the true value is 65cm3. What is the % error?

|14.8-10.4| x100% = 4.4 x100% = 42.31%

10.4 10.4

|58 - 65| x100% = 7 x100% = 10.77%

65 65

Now for Lab!

• You need:– Goggles (cabinet in the back)– Apron (bottom drawers)– Pencil– Calculator