---
title: "Non-carbonate hardness: TH 160, CH 70"
subject: "Water Supply Engineering"
chapter: 4
key: "semester_5/water_supply_engineering:hardness-b78"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/hardness-b78
site: BCT Engineering
---

# Non-carbonate hardness: TH 160, CH 70

Water Supply Engineering · Chapter 4: Quality of Water

- Asked in: 2078 Bhadra, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/hardness-b78
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-4

## Question

The total hardness of water is 160 mg/l and carbonate hardness is 70 mg/l. All the three bi-valent metallic ions causing hardness are same. Determine the non-carbonate hardness, alkalinity and concentration of bi-valent metallic ions.

## Short answer

Non-carbonate hardness is 160 minus 70 = 90 mg/l as CaCO3. Since TH exceeds CH, total alkalinity equals carbonate hardness, 70 mg/l as CaCO3. Using equivalent-weight factors for Ca, Mg and Sr, each bivalent ion concentration is 20.64 mg/l.

## Answer

Total hardness (TH) $= 160$ mg/l, carbonate hardness (CH) $= 70$ mg/l. The three bivalent metallic ions are taken as Ca$^{2+}$, Mg$^{2+}$ and Sr$^{2+}$ present in equal concentration $x$ mg/l (equivalent weights 20.04, 12.15 and 43.81).

#### Non-carbonate hardness
$$
\text{NCH} = \text{TH}-\text{CH} = 160-70 = 90\ \text{mg/l as CaCO}_3
$$

#### Alkalinity
Total hardness is greater than carbonate hardness, so all alkalinity is used in carbonate hardness: alkalinity = CH $= 70$ mg/l as CaCO$_3$.

#### Concentration of the bivalent ions
$$
\begin{aligned}
\text{TH} &= x\left(\frac{50}{20.04}+\frac{50}{12.15}+\frac{50}{43.81}\right) = x(2.495+4.115+1.141) = 7.752x = 160\\
x &= 20.64\ \text{mg/l}
\end{aligned}
$$

**Answer: NCH = 90 mg/l as CaCO$_3$; alkalinity = 70 mg/l as CaCO$_3$; each bivalent ion = 20.64 mg/l.**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. NCH = 160 - 70 =
   A. 90 mg/l
   B. 230 mg/l
   C. 70 mg/l
   D. 160 mg/l
   Answer: A. TH minus CH. (Key point: NCH)

2. Alkalinity equals:
   A. 90 mg/l
   B. 70 mg/l (= CH)
   C. 160 mg/l
   D. 230 mg/l
   Answer: B. TH > CH so alkalinity = CH. (Key point: Alkalinity)

3. Equal concentrations of which ions are assumed?
   A. Na, K, Cl
   B. Ca, Mg, Na
   C. Ca, Mg and Sr
   D. Fe, Mn, Cl
   Answer: C. Equivalent weights 20.04, 12.15, 43.81. (Key point: Three ions)

4. Each ion = 160/7.752 =
   A. 22.0 mg/l
   B. 13.5 mg/l
   C. 30 mg/l
   D. 20.64 mg/l
   Answer: D. x = TH/7.752. (Key point: Ion conc)

## Close variants set in other exams

- [Non-carbonate hardness: TH 150, CH 60](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ahardness-c72) — Water Supply Engineering, chapter 4, asked 2×

## Related in Water Supply Engineering

- [Alkalinity and Ca, Mg: TH 280, CH 75](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ahardness-s73) — Water Supply Engineering, chapter 4
- [Ca and Mg concentration, NCH, alkalinity: TH 400 ppm](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ahardness-bk79) — Water Supply Engineering, chapter 4
- [Ca and Mg concentration, alkalinity (300 mg/l hardness)](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ahardness-ca-mg-equal-300) — Water Supply Engineering, chapter 4, asked 2×
- [Hardness with Ca, Mg, Sr equal, TH 150](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ahardness-as70) — Water Supply Engineering, chapter 4
- [Hardness of well water with strontium 25.26 mg/l](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ahardness-bk81) — Water Supply Engineering, chapter 4