How to calculate new IHD after finding a molecule in IR spec

[AMan24]

Homework Statement

Lets say i have C₅H₁₀O.. The IHD is (5x2+2-10)/2 = 1 IHD

So now i use my IR chart and i determine i have ketone

Now i want to find the new IHD and also figure out what other atoms i have left. So i need to subtract C₅H₁₀O by ketone and figure out the new IHD.

Homework Equations

Given formula: C_cH_hN_nO_oX_x

IHD = 0.5 * [2c+2-h-x+n]

3. The Attempt at a Solution
So for ketone, my guess would be either subtracting CO from the original formula or subtracting C₂H₃O then calculating the new IHD which would be either

C₅H₁₀O - C₂H₃O = IHD 0.5 which doesn't make sense
C₅H₁₀O - CO = IHD 0, which does make sense, but is it correct

And you might say i don't need to calculate it for this problem, but i just want to know for in the future because for some problems finding the new IHD is useful...

My question is: What am i supposed to subtract exactly? And not just for this example but, let's say i have aldehyde, do i just subtract the CO or do i subtract CHO

 

[Borek]

C2H3O

?

 

[AMan24]

?

Isn't that was ketone is? A CO and a CH₃ = C₂H₃O

 

[Borek]

Try to draw a C₂H₃O molecule.

 

[AMan24]

Try to draw a C₂H₃O molecule.

I think i might have figured it out. So for ketones and aldehydes, its just CO. For carboxylic acids and esters its CO2. For amides its CON.. and so on