Which is better: 500g or 500ml of milk?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

21 May 2017

For some strange reason, Kenyan tailors buy clothing materials in metres but take cutting measurements in inches. Similarly, Kenyan surveyors subdivide land using hectares but we trade it in acres.

Even though metres and hectares are metric measurements while inches and acres are imperial standards, they measure the same things – length and area respectively. A big problem arises when two different quantities are used for used for measuring the same thing.

I first noticed this problem in toothpaste: some manufacturers indicate the mass (in grammes) while others state the volume (in millilitres). Last weekend I saw this confusion in milk – there is a brand that is packing it in 500g containers while everyone else uses 500ml.

Unfortunately, the general public assumes that millilitres and grammes are equal. Well, let me be very clear: they are not! The former measures volume while the latter is for mass. I know only one substance for which one ml has a mass of one gramme: this is pure distilled water at 4 degrees celcius, at sea-level.

For all other substances, one millilitre can weigh more than one gramme, or less than one gramme. Cooking oil, for example weighs 0.9g per ml. Honey, on the other hand is 1.33g/ml.

Those two are quite close to water because they are liquids. When we move to solids and to gases, the variations are huge. Iron has about 8g/ml while the oxygen we breathe (at normal temperature and pressure) has just 0.0008g/ml.

After checking the technical information from several manufacturers, I have established that one ml of toothpaste weighs about 1.5g. So clearly, a person buying, say, 100ml is getting a very different amount from the one buying 100g.

The question now is: which is more – 100g of toothpaste or 100ml of toothpaste? In other words, if 100g is priced at, say, Sh200 and 100ml was also priced at Sh200, which of the two gives you the better deal?

To get the answer, let’s think about milk. It is mostly water plus some dissolved and suspended nutrients. We drink milk mainly for the nutrients. Now, one ml weighs about 1.03g. Since one ml of water weighs one gramme, the little extra 0.03g in milk must be from the nutrients.

From 1.03g/ml, it follows that 500ml of milk weighs about 515g. Out of this, 500g is water and so 15g is nutrients. On the other hand, 500g of milk has a volume of about 485ml. In every ml, only there is 0.03g of nutrients. Therefore 485ml of milk has about 14.5g of nutrient.

Clearly then, you get more nutrients from 500ml than from 500g of milk. Thus the former is a better deal if the two are priced the same. Similarly, for toothpaste, you get more value for money when you buy 100ml at Sh200 than 100g at Sh200.

Unfortunately, however, the price of 500ml of milk is not the same as that of 500g – they are from different companies. Therefore, it becomes difficult to make comparisons. For this reason, the Ministry of Industrialisation must wake up and strictly enforce the Weights and Measures Act regarding which quantities are used for trading.