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!

Quote:

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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!

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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.

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

.03 * .18 = .0054

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

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!

Quote:

---

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).

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This makes no sense. If you are given that the sample is 0.030%, how can it be "given as 18%"?

Quote:

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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.

Quote:

---

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.

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!

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