# Finding the mass of an aldehyde

• Chemistry

## Homework Statement:

The mass of an alcohol by the way of hydrogenation of a coresponding aldehyde is 4.45% higher than the mass of the aldehyde. Find the mass of the aldehyde

## Relevant Equations:

Aldehyde + H2 = alcohol
Ethanal + H2 = ethanol
Aldehyde + H2 = alcohol
Ethanal + H2 = ethanol
This is all that I have done so far

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.Scott
Homework Helper
4.45% of the mass of the aldehyde is the mass of what you are adding to the aldehyde.
All that's left is the Algebra.

4.45% of the mass of the aldehyde is the mass of what you are adding to the aldehyde.
All that's left is the Algebra.
In hydrogenation, we add only hygrogen
So meaning we are adding 4.45% mass of the aldehyde worth of hydrogen. Let me try

4.45% of the mass of the aldehyde is the mass of what you are adding to the aldehyde.
All that's left is the Algebra.
Let's say x=100
X+H2 = 100+4.45
100+4.45=104.45
This is what I got. I'm sort of lost here

I sort of get this question as have an x g of aldehyde
Add 4.45% of x g of hydrogen to it and we get 4.45% g higher alcohol.
But I'm stuck with the algebra

.Scott
Homework Helper
Let's say that X is the mass of the aldehyde.
X+mass(H2) = what function of mass(alcohol)?

Let's say that X is the mass of the aldehyde.
X+mass(H2) = what function of mass(alcohol)?
X+2=(4.45/100) x (X)+X
Is this ok?
We could say x=100
100g + 4.45g= 104.45g

Last edited:
Borek
Mentor
Assuming you used 1 mole of hydrogen (what was its mass?) what has to be the molar mass of the aldehyde?

chemisttree
Homework Helper
Gold Member
The mass referred to here is molar mass. Going from alcohol to aldehyde represents a loss of two hydrogens. Let X be the molar mass of the aldehyde and X+2 be the molar mass of alcohol.

The ratio of (X+2)/X = what?

molar mass alcohol / molar mass aldehyde = what? in this example...

The mass referred to here is molar mass. Going from alcohol to aldehyde represents a loss of two hydrogens. Let X be the molar mass of the aldehyde and X+2 be the molar mass of alcohol.

The ratio of (X+2)/X = what?

molar mass alcohol / molar mass aldehyde = what? in this example...
0.0455
When that is multiplied with 100 we get 4.45%

chemisttree
Homework Helper
Gold Member
Almost. The value of that ratio is 1 + 0.0445 or 1.0445

Do you see that if the numerator is greater than the denominator the result must be > 1?

Solve for X

Almost. The value of that ratio is 1 + 0.0445 or 1.0445

Solve for X
Yes because it's an increase right?

X+2/x = 1.0445
X+2=1.0445x
1.0445x-x=2
0.0445x=2
X=2/0.0445
X=44.94

chemisttree
Homework Helper
Gold Member
That molar mass corresponds to which aldehyde? Consider only C,H&O in the formula.

That molar mass corresponds to which aldehyde? Consider only C,H&O in the formula.
The lowest formula of aldehyde is HCOH
Mass of HCOH= 30
44.94/30=1.498

chemisttree
Homework Helper
Gold Member
Nope. Keep trying. This formula wt is not 30... it is ~45.

Last edited:
Nope. Keep trying. This formula wt is not 30... it is ~45.
CnH2n+1CHO
So CH3CHO mass = 44
44.94/44=1.02

chemisttree
Homework Helper
Gold Member
Close enough!

jim mcnamara
Close enough!
Close enough?
What was the error here?

chemisttree
Homework Helper
Gold Member
There was no error on your part. Sometimes the data we get from an experiment isn’t perfect, isn’t exact. The data from this example was only off from theoretical by 2%. That’s usually good enough. Close enough!

BillTre
There was no error on your part. Sometimes the data we get from an experiment isn’t perfect, isn’t exact. The data from this example was only off from theoretical by 2%. That’s usually good enough. Close enough!
Thank you very much
You teaching technique is awesome. I really learnt a lot.
PS. There is one more question that I have posted could you take a look at that too please