[nurse2b]

 Q: A radiator is to be protected against freezing to -20F. Calculate the volume of ethylene glycol C2H4(OH)2 need per liter of water. Density of glycol os 1.11 g/ml.


I know to convert F to C which is 6.7C.
Next find how many moles of ethlyene glycol. this is where my brain has frooze. Any input will be appericated.

[sdekivit]

the formula for freezing point depression is:

delta T_f = K_f * c_m * i

i = 1 since there is no dissociation ( van 't Hoff's factor)

delta T = 28,888888889 degrees C and the cryoscopic constant of water  is -1,86 degrees C / molal.

So we get:

-28,888888889 = -1,86 * c_m which leads to a molal concentration of 15,53166069 molal ethylene glycol.

this means that there is 15,53166069 mol ethylene glycol / kg water. We know that 1 L water weighs 1 kg so we need 15,53166069 mol ethylene glycol

that's 15,53166069 * 62,068 = 964,0191157 g ethylene glycol and that's 964,0191157 / 1,11 = 8,7 x 10²ml ethylene glycol.

[Borek]

15,53166069

Have you ever heard of significant digits?

[sdekivit]

Have you ever heard of significant digits?

rouniding of in the calculation is forbidden. You may only round off in significant digits at the end of your calculation, which i did

[lemonoman]

Agreed.  I personally like using doing the algebra first, and plugging in numbers last, than manipulating the numbers mid-question (unless of course, it's a HUGE question that need to be broken down into parts).

I have a friend who writes out every single mathematical calculation, like if there was,

X = (234)(12-y)/6

They'd write:

X = (234/6)(12-y)
X = 39(12-y)
X = (39)(12) - (39)(y)
X = 468 - 39y

And it annoys the H*LL out of me, when they've got this HUGE quadratic equation and they solve it step-by-step...P.S. I know they can do it, they just like to be ABSOLUTE SURE...but come test-time, it shoots them up the rear end, I tell ya.

Something to think about.

[Borek]

rouniding of in the calculation is forbidden. You may only round off in significant digits at the end of your calculation, which i did

I am not asking you to round down intermediate results - do the calculations using as much significant digits as you want. But when you state delta T = 28,888888889 it is just plain wrong, as according to the question data delta T = 29 deg C. Don't mistake intermediate value displayed by the calculator with the real physical value that has finite accuracy.

[xiankai]

then round them off to 4 sig. fig, as the numbers in the lower digits cannot be elevated to a degree that affects the end reading.

for eg, i'll use 28.89 / 1.86= 15.53

15.53 x 62.07 = 963.94

963.94 / 1.11 = 868.41

round it to 2 sig. fig. like u did, and u'll get the same answer. now isnt that more simple

and oh please god dont replace dots with commas. for a moment i had a heart attack seeing your delta T value at an astronomical value.

[sdekivit]

then round them off to 4 sig. fig, as the numbers in the lower digits cannot be elevated to a degree that affects the end reading.

for eg, i'll use 28.89 / 1.86= 15.53

15.53 x 62.07 = 963.94

963.94 / 1.11 = 868.41

round it to 2 sig. fig. like u did, and u'll get the same answer. now isnt that more simple

and oh please god dont replace dots with commas. for a moment i had a heart attack seeing your delta T value at an astronomical value.

yeah sorry, here in holland we use comma's instead of dots . And yeah it's is much easier, but i had several question were it did make a difference. For example a buffersolution where u need to calculate the volume NH3-gas needed to bring the buffer at pH = 9,2 with rounding off the question had the answer 44 L and with rounding off the answer was 49 L.

So in my replies you'll see a lot of digits and at the end i round off. Sorry i learned that and i stick to that.

[Borek]

So in my replies you'll see a lot of digits and at the end i round off. Sorry i learned that and i stick to that.

Are you aware of the fact that this way you are teaching others wrong approach?

[sdekivit]

Are you aware of the fact that this way you are teaching others wrong approach?

strictly seen is the conversion of Fahrenheit to Celcius the calculation. I am thought intermediate rounidng off is wrong. The final answer must be given in the amount significant digits of the given that has the lowest amount of significant digits.

I know the common method is to give 2 extra digits as the siginificant numbers to avoid rounding-errors.

[Borek]

I am thought intermediate rounidng off is wrong. The final answer must be given in the amount significant digits of the given that has the lowest amount of significant digits.

Nobody asks you to round intermediate values in your calculations. Use them as they are displayed on your calculator. However, when you copy intermediate value to your post and you write something like 15.9132567983476983245 g you are plain wrong and you are teaching others bad habits.

[sdekivit]

Nobody asks you to round intermediate values in your calculations. Use them as they are displayed on your calculator. However, when you copy intermediate value to your post and you write something like 15.9132567983476983245 g you are plain wrong and you are teaching others bad habits.

i've posed the question to some physicists i know and they say:

it's better to give a intermediate answer in too many digits than too less. Only at the end you must round of in the correct significant digits.