In a medical alcohol solution, the mass ratio of pure alcohol to water is 3:4. How many grams of pure alcohol are needed to prepare 420g of this medical alcohol solution?

3 + 4 = 7
420*3/4=315g

The ratio of pure alcohol to distilled water in medical alcohol is 3:1. Prepare 5000 ml

Distilled water 5000 (3 + 1) = 1250 ml
Alcohol 5000-1250 = 3750 ml
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A kind of medical alcohol is made of pure alcohol and water in the mass ratio of 73:27. How many kilos of pure alcohol is needed to prepare 500kg of this kind of medical alcohol?
With the equation, there is a process

Suppose pure alcohol x kg
X：500-X=73：27
73(500-X)=27X
73*500-73X=27X
100X=36500
X=365
There are 365 kilograms of pure alcohol

A treatment room needs to prepare 1000 ml of disinfectant alcohol with a volume fraction of 0.75. How many ml of medical alcohol with a volume fraction of 0.95?

According to the amount of alcohol unchanged! Set the need for 0.95 alcohol XML
75 · 1000 = 0.95 · X
The result is x = 789.5ml

In the hospital, doctors usually use 70% alcohol solution to disinfect patients. If 70 ml alcohol and 30 ml water are mixed to prepare medical% B
In hospitals, doctors usually use 70% alcohol solution to disinfect patients. If 70 ml alcohol and 30 ml water are mixed to prepare medical alcohol, the solute mass fraction of the solution is -- (the density of alcohol is 0.8 g / cm3)

There is something wrong with this question
It means 100% alcohol and water, but actually there is no 100% alcohol. The so-called pure alcohol is 95%
If it's 100%, the answer is 65.12%

How to dilute 75% alcohol to 30% alcohol

Add water
(30-0):(75-30)=30:45=2:3
Two parts of 75% alcohol and three parts of water give you 30% alcohol

How to dilute 95% alcohol to 70%
What's the ratio of 95% alcohol to water (volume ratio)? What's the ratio of 95% alcohol to water (mass ratio or volume ratio)?

Generally speaking, it is volume ratio. If it is written like this, the requirements are not so strict. After all, the relative density of alcohol and water at different temperatures does not seem to be the same. Basically, 100 ml of 95% alcohol can be mixed with 36 ml of water
100 ml of 95% alcohol contains 95 ml of pure alcohol and 5 ml of water
If 70% 95 ml alcohol accounts for 7 parts in the whole, water accounts for 3 parts, 95:7 = milliliter of water: 3
The milliliter of water is about 41 milliliter,
Then subtract 5 ml from 95 for the amount of water added

Alcohol dilution
If you want to buy 75% medical alcohol, you can only get 95% at the drugstore. Now you want to dilute it to 75% with a total of 500ml,

(1) The following formula can be used to calculate the volume of concentrated alcohol:
75%*500ml=V*95%
V=0.75*500/0.95
=394.7ml (volume of 95% alcohol required for 500ml 75% alcohol)
The mass of sterile water is 500-394.7 = 103.5ml
(2) If 500 ml of 95% alcohol is used, the volume of 75% alcohol can be prepared is as follows:
500*95%=75%*V
V=500*0.95/0.75
=633ml
The quality of sterile water is 633-500 = 133ml
(when high concentration alcohol is diluted with water, the volume is slightly smaller than the calculated volume, but the effect is small.)

How to dilute 95% alcohol to 75% alcohol

There is alcohol solution x, in which the pure alcohol is 0.95x
After diluted to 75% alcohol, the pure alcohol does not change, so the diluted alcohol solution is 0.95x/0.75
So the dilution method is 0.95x/0.75-x = 4x / 15
That is, add 4 / 15 water of the original alcohol solution

How many grams of distilled water do you need to add to prepare a 64% alcohol solution with 800g pure alcohol?
Let x calculate

64%(x+800)=800
x=450

In a medical alcohol solution, the mass ratio of pure alcohol to water is 3:4. How many grams of pure alcohol are needed to prepare 420g of this medical alcohol solution?