# This Cannot Be Density In Chem

1. Apr 4, 2005

### dagg3r

The density of a substance is 1.37 g/cm3. Convert this into kg/m3.
Remember that: 1kg = 103 g and 1 m = 102 cm. Express your answer to 1 decimal place (don't include units).

what i did is

1.37g = 0.00137 kg
1cm^3 = 1*10^-6 ie (1/(10^3))

0.00137 / (1*10^-6) = 1370.0 kg/m^3 is that correct?

i dont think so looks so large hehehe maybe im confused heh someone shed some light on that
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question 2
What mass of K2CO3 is needed to prepare 200 mL of a solution having a potassium ion concentration of 0.150 M? (molecular mass of K2CO3 = 138.2 g mol-1)
Hint: remember that 2 K+ ions are produced in the solution, so twice the conc of the original salt)

its a multiple choice answer i did the working out and got 2.07g is that correct???

ill show whawt i did
n(k2c03)=0.150*(200/1000L)=0.03 mol /2 = 0.015mol
0.015 * 138.2gmol^-1 = 2.0703g

i hope thats right please tell me if it is wrong or right ppl thanks

2. Apr 4, 2005

3. Apr 4, 2005

### DaveC426913

"Remember that: 1kg = 103 g and 1 m = 102 cm. "

What??? Is this like saying "pi=3"?

Oh! Oh. Of course, you meant 1kg=10^3g and 1m=10^2cm.

Check you answers by estimation. Water is 1g per cubic cm. 1 cubic meter of water is 1000kg. Do your answers fall in line?

4. Apr 5, 2005

### dagg3r

so i assume my answer is wrong???? since i had 1cubic centimeter nearly 1000kg??? so how would i correctly do the density question some one please show me thanks

5. Apr 5, 2005

### ZapperZ

Staff Emeritus
OK, I'll spend some time doing this because there is a very simple technique in doing such conversion, and dimensional analysis is such an important, but simple part of chemistry (and physics, and engineering), that this should be taught clearly.

What you need to do is actually multiply by 1! I'll explain. You have

$$1.37 \frac{g}{cm^3}$$

You need to mulply that by "1" and handle the units as if they are like any algebraic symbol. The "1" that you are going to mutiply is the conversion factor. I will first do the numerator, which is "g" as such:

$$1.37 \frac{g}{cm^3} \frac{1 kg}{1000 g}$$

I multiplied that with 1kg/1000g which actually hasn't changed anything. That fraction is really just "1", only it is expressed in different units. Notice now that the unit "g" cancels out just like any algebraic units, leaving you now, if you do the numerical calculation, with a quantity having the units kg/cm^3. So now let's deal with the cm^3 unit. I can again multiply with another "1", since I know that 1m = 100 cm. Thus

$$1.37 \frac{g}{cm^3} \frac{1 kg}{1000 g} \frac{100*100*100 cm^3}{1 m^3}$$

Again, note that cm^3 cancels out, leaving you with m^3. You now have a number in units that you want.

Again, the technique here is to do this ONE step at a time, and find the connection between the necessary units (i.e. 1 kg = 1000g, 1m = 100 cm), and use those relationship to form a fraction that is essentially "1". Then multiply that to the number you want to convert. Note that in some cases, you have to do this is more than just one, or two, or three, or more steps. As long as you keep the conversion straight, and make sure the units cancel out until you get the right units, this technique should work all the time.

Zz.

6. Apr 5, 2005

### DaveC426913

What do you mean wrong? Your answer was 1370kg per cubic metre.