# Thread: Help with percentages....

1. ## Help with percentages....

Hi,

I've been doing some chemical analysis on concrete to determine the chloride content. I've found that a 5g concrete sample is made up of 0.030% chloride. I now need to determine the percentage chloride per weight of cement, which is given as 18% of the concrete (i.e. 0.9g).

I've done the following calulation but I'm not sure if it's correct:

100 / 18 = 5.555555555555556

5.555555555555556 X 0.030 = 0.1666666666666667

And I've taken that last figure as the answer. It seems straightforward but I'm not sure....

I hope I've explained that properly, any help is much appreciated!

2. ## Re: Help with percentages....

Originally Posted by mjcapri
Hi,

I've been doing some chemical analysis on concrete to determine the chloride content. I've found that a 5g concrete sample is made up of 0.030% chloride. I now need to determine the percentage chloride per weight of cement, which is given as 18% of the concrete (i.e. 0.9g).

I've done the following calulation but I'm not sure if it's correct:

100 / 18 = 5.555555555555556

5.555555555555556 X 0.030 = 0.1666666666666667

And I've taken that last figure as the answer. It seems straightforward but I'm not sure....

I hope I've explained that properly, any help is much appreciated!
It's probably best to set this up as a ratio. You know that for every 5g of concrete, of that 18% (i.e. 0.9g) is cement and 0.03% (i.e. 0.0015g) is chloride.

So the ratio of cement to chloride is 0.9 : 0.0015 = 9 : 0.015 = 1 : 0.0016666667.

So for every gram of cement, there is 0.00166666667 grams of chloride.

You were close, but I think you had the answer as a percentage rather than as a weight.

3. ## Re: Help with percentages....

.16666..... is greater than .03 ; seems to me should be lesser...

.03 * .18 = .0054

But not positive: your problem is difficult to follow...

4. ## Re: Help with percentages....

Thanks guys that was really quick!

Prove It - I actually want the answer as a percentage, if that helps.

Wilmer - I know, I'm pretty sure it is straightforward but I've had difficulty explaining it to everyone!....

Here's my logic if it helps:

There's 0.030% chloride in the original 5g.

There's 18% (0.9g) of cement in the original 5g.

To get the % chloride by weight of cement, there is still 0.030% chloride but instead of being of 5g (100%) it's now of 0.9g (18%), so I've multipied the 0.030% by the number of times 0.9g (18%) goes into 5g (100%). I think what confuses people is the amount of chloride, be it by weight or percentage, stays the same - it's what it's a percentage of that has changed.

Thanks again guys!

5. ## Re: Help with percentages....

Originally Posted by mjcapri
Hi,

I've been doing some chemical analysis on concrete to determine the chloride content. I've found that a 5g concrete sample is made up of 0.030% chloride. I now need to determine the percentage chloride per weight of cement, which is given as 18% of the concrete (i.e. 0.9g).
This makes no sense. If you are given that the sample is 0.030%, how can it be "given as 18%"?

I've done the following calulation but I'm not sure if it's correct:

100 / 18 = 5.555555555555556
18% means 18/100, not 100/18.

5.555555555555556 X 0.030 = 0.1666666666666667

And I've taken that last figure as the answer. It seems straightforward but I'm not sure....

I hope I've explained that properly, any help is much appreciated!
It's impossible to understand what the question really is. Are you saying that "a 5g concrete sample is made up of 0.030% chloride." and you want to determine the amount of chloride (not percentage) in 18% of the sample?
18% of 5g is 5(.18)= .9 g. 0.030% of .9 g is .9(.0003)= 0.0027 g.

6. ## Re: Help with percentages....

It's actually the percentage I need.

It will maybe help if I explain that the concrete is made up of cement, aggregate and water. The chloride will be contained (as a contaminant) within either the aggregate or water. I'm assuming (based on government guidance!) that the ratio of cement to the other two constituents is 18%.

So, there is X amount of chloride in the concrete - that amount doesnt change, it's always there. I know that it is 0.030% of the 5g sample (the weight isn't important unless is helps with the calculation).

Whats important is the chloride to cement ratio, the latter being 18% of the 5g sample.

The answer has to be geater than the original 0.030% as it's the same amount of chloride but in relation to only 18% of the total concrete constituents.

Thanks again!

7. ## Re: Help with percentages....

Get 18% of the mass of concrete, this is the mass of cement.

get 0.03% of the cement, this is the mass of chloride

If this is a real sample I would expect this to be a small number - I make it 27mg, but you should use more sig figs in your initial concrete mass if this is a real test.

for more help on percentages check out Free Online Tutoring | Maths Help | chemistry Help Exam revision | My tutor | My tutoring - the bit on percentages is under the maths section!

Just reread your post - 18% of 5g concrete is mass of cement
0.03% of 5g is mass of chloride

percent chloride in cement is the second value over the first time 100