# mass ratio question

#### psoke0

Joined Mar 31, 2017
174
guys i read something and i dont have much knowladge can you help me ?
its says carbon black content ( mass ratio ) of 0.3% what does that means ? if i want 250 grams of some substance(PbO2) what is the mass ratio of 0,3 % in grams carbon black.

for example if i want to know how much carbon black i should put in in grams with 0,3 % mass ratio in 250 grams total (with Carbon black added) of PbO2 is that means carbon black grams = ( 0,3 % x 250 g ) / 100 ? is it means 0,75 grams of carbon black should be added to 249.25 grams of PbO2 ?

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#### MrChips

Joined Oct 2, 2009
24,170
guys i read something and i dont have much knowladge can you help me ?
its says carbon black content ( mass ratio ) of 0.3% what does that means ? if i want 250 grams of some substance(PbO2) what is the mass ratio of 0,3 % in grams carbon black.

for example if i want to know how much carbon black i should put in in grams with 0,3 % mass ratio in 250 grams total (with Carbon black added) of PbO2 is that means carbon black grams = ( 0,3 % x 250 g ) / 100 ? is it means 0,75 grams of carbon black should be added to 249.25 grams of PbO2 ?
Sounds right.

#### psoke0

Joined Mar 31, 2017
174

#### Papabravo

Joined Feb 24, 2006
16,775
We're electronics engineers, not chemists. You pays your money and you takes your chances. Like the man said: "it looks right". That does not mean it is correct. If you can arrange for the right number of moles of each substance to be present in the final mixture, that is if there are no chemical or physical constraints on doing this, then it has a higher probability of being correct.

C = 12 g/mole
O = 16 g/mole
PB= 207.2 g/mole

How many moles of each do you need to get 250 grams of mixture?

#### psoke0

Joined Mar 31, 2017
174
i think i need 1 mole of carbon 2 moles of O and 1 mole of PB ?

#### Papabravo

Joined Feb 24, 2006
16,775
i think i need 1 mole of carbon 2 moles of O and 1 mole of PB ?
That comes to 251.2 grams -- a bit heavy. Which is why I was suspicious of the original answer, and suggested seeking an alternative way of doing the calculation.
I get .0625 moles of carbon black and 1.042 moles of PbO2, which can then be shown to have the correct mass ratio.

.0625 * 12 = 0.75 grams
1.042 * 239.2 = 249.246 grams
0.75 + 249.246 = 249.996 grams

Now it seems that the intuitive answer was correct and also has an additional bit of evidence behind it.
This happens because there are no constraints on mixing the two substances.

In chemistry you will find that unexpected constraints will pop up and prevent you from creating arbitrary combinations. It pays to have a backup plan.

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#### psoke0

Joined Mar 31, 2017
174
That comes to 251.2 grams -- a bit heavy. Which is why I was suspicious of the original answer.
so i need 0.0625 mole of carbon then

#### jpanhalt

Joined Jan 18, 2008
11,088
There is some ambiguity in the term "mass ratio." Does it mean molar ratio or weight ratio? In general on Earth, when the word mass is used it used synonymously with weight -- at least by the public.

I suspect in this case, it means weight ratio or ratio by weight which is often written: "wt/wt." Had the author meant mole ratio, it should have said so.

Wikipedia seems to concur: https://en.wikipedia.org/wiki/Mass_fraction_(chemistry)

#### psoke0

Joined Mar 31, 2017
174
There is some ambiguity in the term "mass ratio." Does it mean molar ratio or weight ratio? In general on Earth, when the word mass is used it used synonymously with weight -- at least by the public.

I suspect in this case, it means weight ratio or ratio by weight which is often written: "wt/wt." Had the author meant mole ratio, it should have said so.

Wikipedia seems to concur: https://en.wikipedia.org/wiki/Mass_fraction_(chemistry)
okay so mass % is M% = MC/MS * 100 MC = mass of compound MS = mass of total mixture so
MC = MC% (which in my case is 0,3 % ) x MS (250 grams) / 100 which comes to 0,75 grams of mass of compound whic is carbon black in my case right ?

#### psoke0

Joined Mar 31, 2017
174
That comes to 251.2 grams -- a bit heavy. Which is why I was suspicious of the original answer, and suggested seeking an alternative way of doing the calculation.
I get .0625 moles of carbon black and 1.042 moles of PbO2, which can then be shown to have the correct mass ratio.

.0625 * 12 = 0.75 grams
1.042 * 239.2 = 249.246 grams
0.75 + 249.246 = 249.996 grams

Now it seems that the intuitive answer was correct and also has an additional bit of evidence behind it.
This happens because there are no constraints on mixing the two substances.

In chemistry you will find that unexpected constraints will pop up and prevent you from creating arbitrary combinations. It pays to have a backup plan.
thanks

#### Papabravo

Joined Feb 24, 2006
16,775
so i need 0.0625 mole of carbon then
Yes, which is the same as 0.75 grams of mass or (7.35 e-3 Newtons of weight)
This only works because there are no restrictions on mixing arbitrary amounts of the two substances. This may not always be true.

One place where this caution is required is solutions of water and acid. It is not possible to have arbitrary mass ratios depending on the particular type of acid. Trying to do this can get you killed instantly. For example: using an eye dropper of water to dilute a concentrated acid.

#### Papabravo

Joined Feb 24, 2006
16,775
Would it be impertinent to ask why you are making this stuff?

#### jpanhalt

Joined Jan 18, 2008
11,088
A mass fraction would be 0.3g of carbon black + 99.7g of PbO2. Thus the mixture would be 0.3g carbon black per 100g total, which is probably close enough to 0.3g of carbon black to 100g of PbO2 not to matter.

#### psoke0

Joined Mar 31, 2017
174
Would it be impertinent to ask why you are making this stuff?
i dont put water to acid i do it other way around and i do use full protection safety goggles and gloves . im making this for experiment see if pbo2 with mixture of carbon black will hold up in 30 % diluted sulfiric acid . because when i put only pbo2 its breaks down in solution so maybe carbon black will be used as binding agent to hold it together

#### Papabravo

Joined Feb 24, 2006
16,775
i dont put water to acid i do it other way around and i do use full protection safety goggles and gloves . im making this for experiment see if pbo2 with mixture of carbon black will hold up in 30 % diluted sulfiric acid . because when i put only pbo2 its breaks down in solution so maybe carbon black will be used as binding agent to hold it together
I understand. I wasn't implying that you might do certain silly things it was mostly for the other potential readers of the thread who might otherwise pay less attention to certain warnings. I think we're good.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,068
I hate chemistry. I was trained to use concentrated HF. Hydrofloric acid (Very very very nasty stuff).

#### MrChips

Joined Oct 2, 2009
24,170

#### Papabravo

Joined Feb 24, 2006
16,775
I hate chemistry. I was trained to use concentrated HF. Hydrofloric acid (Very very very nasty stuff).
Roger that -- just about the nastiest, readily available stuff, short of Plutonium.

#### psoke0

Joined Mar 31, 2017
174
I understand. I wasn't implying that you might do certain silly things it was mostly for the other potential readers of the thread who might otherwise pay less attention to certain warnings. I think we're good.
okay thank you for your help